besides, bitcoin can also advance and change. it is not like the design is absolutely unchangeable. it is not significantly changed so far because there is nothing better to change to. which means even if a much better solution were found, bitcoin may be the one finding it and using it first!
I'd like to think that too. However, if I'm being honest, that's sort of at odds with Bitcoin's very slow and methodical development culture. It's also at odds with these words from Satoshi:
The nature of Bitcoin is such that once version 0.1 was released, the core design was set in stone for the rest of its lifetime.
Let's imagine proof-of-stake was actually solved, and reasonably secure. Do you think Bitcoin would switch to it? I don't.
The same goes for things like total supply and emission rate too. We don't know that a firmly limited supply is the optimal route in terms of long term mining security; it's experimental at this point. But either way, do I see Bitcoin hard forking to increase the 21 million cap? No.