you make good points.
Could one buy a smart TV and use the internal device until such time than an external device has more features than the internal SMART TV and then later use an external box in tandem with a SMART TV? Then one would get some benefit for a while of a single remote
If you are simply going to use netflix or amazon, there may not even be a need for an external device. However, what happens to most people (at least that I know of) is that as their knowledge and experience on the subject expands, they begin to realize there are more and more options (like having a home video streaming device for example), you will start wanting to add more stuff. The more options you have, the less dependent you are on rip-off cable or satellite services. Ultimately, you may even end up with a home media center PC like I did.
So, to the point, no... it's not necessary to jump all-in and start buying external devices. I don't follow the market so I don't know how much more expensive a "smart" TV is than a regular LCD with a few HDMI ports. If you are a smart and patient shopper, I suppose you could make it work without too much trouble and extra expense.
As far as Tv technology goes, I'm not really into the latest high refresh, fastest gizmo 4k TV blabla stuff, regular old HD TV @ 1080 is more than sufficient for me. That being said, I like the LED TV's rather than LCD because, and this may sound silly to a lot of people, the LED TV's don't put off the massive amount of heat like the older LCD TV's do.
I know I'm getting off topic now, but my uses for entertainment aren't to the level of home theater nut. I just like to have a lot of options and variety for sources of media... so adjust accordingly if you are the type who likes the latest and greatest.