The idea with the constantly moving flame front is that it spreads the heat out. The limitation with aerospikes is getting enough coolant through the spike. Bells are simpler to cool, which as I understand more than makes up for them needing more cooling.
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.
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.