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. . . it’s meant to be a proof-of-concept.

Yes. I'm sure all they want to do is get high and fast enough to show that they can make the aerospike work, and get some data on performance.

Getting to orbit is all about speed. You can get high with low speeds (which, for orbiting spacecraft, can be something around Mach 2). Orbit requires about Mach 20. So the real issue is getting high enough to reduce drag, then go fast. Back when we had the Space Shuttle, if you watch the films you see that they got to Mach 1 at less than one minute into the (almost) nine minute main burn.

Most of that main burn was spent high enough that the bells on the engines were very efficient. Picking up a little efficience (with the aerospike) at Mach 3-4, and losing it from Mach 4 to Mach 20 doesn't sound like a very good optimization.

But I'm all for letting them try, particularly since NASA is not involved. The NASA of nowadays (all DEI and government bureaucrats) has ruined anything manned for the last 30 years.

The Show Enterprise, I believe, had this in their intro. A plane that hits high orbits and costs around the earth before coming to a landing wherever it destined.