Most all novel ideas seem ordinary once the technology that makes them work becomes commonplace an well understood.
The lightbulb was a pretty novel invention. However, once you understand what a vacuum is and how keeping oxygen away from a filiment will allow it to glow without burning up, it seems pretty obvious.
However, it was a novel idea at the time.
To use an analogy from the early 1700's, it was obvious to many people that if a ship had a clock that could maintain home-port time within a couple minutes' accuracy, without being affected by the movement of the ship, one could use the difference between ship time and home-port time to compute longitude. Nobody thought such a clock could be constructed, but it was obvious to everyone that, if it were, one could compute longitude with it.
Patent law handles that situation just fine. If something is commonly known it's not patentable. The usuall method of proving something was commonly known is to present prior art such as writings about the topic, but witness testimony can also be given.
Many of today's patent cases could be regarded as being equivalent to someone in 1710 patenting the computation of longitude via timekeeper (without any clue how the timekeeper should be constructed), and then suing John Harrison for patent infringement when he invented a clock that actually worked for that purpose.
If they were the first to come up with the idea of using such a clock to measure longitude, they would have the ability to patent it for 17 years.
In 1728 when John proposed his idea for a Marine clock, the patent on creating such a clock would have alreay have expired.
However even if it had not expired, his ideas for ides for how the clock was made were in themselves novel, and could have been patented. Many patents include references to other patents. Patents protect the inventions of people while also allowing the inventor to make their inventions public.
They give inventors a way to make money off of thier inventions while also encourage sharing those ideas once they are protected, which helps encourage others to invent.
In other words someone figures out that they could figure out longitude with such a clock, but they can't figure out how to make the clock. They patent the idea to to protect their part, and they can then share that idea and with others to try and find other people capable of actually building the clock. Otherwise they might not tell anyone else about their idea because they are afraid the person who can figure out how to build the clock will make all the money off of the idea.
Although it should be possible to patent things before all the technologies to make them work are fully practical, the test of novelty for such patents should be stricter than for inventions which are practical from the beginning.
That simply discourages people from sharing their ideas. If the idea is novel when they come up with it, then it's a novel idea. People are already discouraged from patenting and sharing novel ideas that they can't implement because of the time limitation on patents, your idea would discourage people even more.
You idea is also unimplementable. How do we know what is going to be obvious in the future once technology that doesn't exist yet is common?
If anything we should be encouraging people to patent ideas that are not yet practical, because it brings out new ideas and stimulates others to find ways of making them practical.
The person who finds a way to make it practical also invents something novel by making it practical, and has their own invention to patent, so they don't get left with nothing either.
But suppose someone had patented the idea of making a lightbulb using a filament of a material that wouldn't vaporize (but did not specify what that material should be).
Patent law handles that situation just fine. If something is commonly known it's not patentable. The usuall method of proving something was commonly known is to present prior art such as writings about the topic, but witness testimony can also be given.
Suppose someone had, prior to the development of Viagra, patented the concept of correcting erectile dysfunction by using drugs to dialate certain blood vessels (without specifying what drugs should be used). Should such a person have been able to sue Pfizer for patent infringement?
If they were the first to come up with the idea of using such a clock to measure longitude, they would have the ability to patent it for 17 years.
In 1728 when John proposed his idea for a Marine clock, the patent on creating such a clock would have alreay have expired.
The analogy isn't perfect. Suppose John Harrison had managed to build his clock faster. Should he have had to pay patent royanties?
That simply discourages people from sharing their ideas. If the idea is novel when they come up with it, then it's a novel idea.
"Novel" ideas which lack practical application are a dime a dozen. Thomas Edison noted IIRC that invention is 10% inspiration and 90% perspiration (or maybe he said 1%/99%--the latter figure would probably be more accurate in any case). Overly-loose patent standards focus too much reward on the "10%"; if something is not done, the long-term result will be that inventors will focus their efforts on recording vauge ideas for which nobody will develop any practical application.
You idea is also unimplementable. How do we know what is going to be obvious in the future once technology that doesn't exist yet is common?
In many of the more egregious cases, by looking for analagous uses of existing technologies. Suppose someone finds out that a company is planning to release a refrigerator powered by replaceable metal hydride bottles; should a person be allowed to patent the concept of using a hydride-powered refrigerator to cool lemon-lime soda, and demand royalties from anyone useing it to keep that particular beverage cold? If the hydride power refrigerator wasn't well-known prior to the person seeking such a patent, he may well be the first person to think of using it for that particular purpose. I would argue, however, that the use of refrigerators in general to cool many beverages including lemon-lime soda was widely understood; even if nobody else had thought to use that particular kind of refrigerator to cool that particular beverage, the "invention" would have been thought of the moment it would have been useful (i.e. as soon as somebody who happened to have that sort of fridge happened to have some lemon-lime soda he wanted to cool).
Incidentally, very shortly after the Pentium FDIV fiasco, I pondered the problem of long division and happened to independently develop a practical method of rapid division which it turned out Cyrix had just patented (I did a patent search after confirming that my method worked). I don't know if any programs like PGP use the method, though it would be suitable for both hardware and software implementations (and would be quite excellent for something like PGP). Even though the idea behind that patent might not have been "obvious", I would guess that the inventor and I aren't the only two people to have thought of it.