I wonder if aerospikes were popular as an idea in the J-2 engine ~Apollo era since USA only had gas generator engines (and expander RL-10).

You can't get to very high chamber pressures with those, and then maybe aerospike was a way to work around the limitations.

Then XLR-129 and SSME came along with staged combustion cycle and you can just have higher pressure engines. They can both run at sea level and still have a decent efficiency in vacuum.

The linear aerospike for X-33 was kind of a neat tech demo and fit in with the whole shape of the vehicle and composites, non-tubular tanks and large base area. Maybe too many new things at once in retrospect.

The estimates are that a fully functional RDE will be up to 25% more efficient than a standard bell/deflagration design.

Efficiency means less heat per kg of mass to orbit. And with the rocket equation, that’s a lot less engine per kg of payload.

Efficiency means either less heat, or intensely more incentive to solve the heat problem even with unobtanium. You could make the damned things out of molybdenum, which has 8 times the thermal conductivity of titanium, and a higher melting point.