Are we stuck with Bitcoin as is forever, and the only scaling solutions must be built on top of the current utterly fixed ruleset?
Probably.
I agree. Likewise, given the difficulty of changes now as shown by segwit and block size changes, any further changes will be even more difficult to get consensus. And this isn't necessarily a bad thing, changes should be very difficult to make barring something catastrophic (e.g March 2013). Think about changing tcp/ip.
immutability is bullish af, just look at btc price now.