How long can Bitcoin last?

I have heard everyone say that Bitcoin is here to stay, but as a crypto noob I have a couple questions as to possible ends to Bitcoin.

First, there can only ever be ~21 million Bitcoin however, as we all know, if you lose access to your wallet you lose your coins forever. From what I've read on this sub, Satoshi's wallet has around 1 million Bitcoin in it which may never be activated again. Another post I saw mentioned the FBI has some wallet with roughly 600k Bitcoin from someone they arrested. Combined this is almost 10% of the total Bitcoin supply that will ever exist lost. Add to that the random anecdotes where someone lost access to 1000 Bitcoin here or 100 Bitcoin there, and I wonder how long before most if not all Bitcoin is just permanently lost. Do you think it will ever lose so much supply that it becomes unusable or will there be some point where its no longer lost in significant quantities that it will essentially last forever?

Second, as I understand it, PoW requires 51% of the computational power to be working honestly for us to trust the network. Is it possible for someone like Google or Microsoft or possibly even a governing body like the US or China to use some supercomputing databanks or quantum computers to essentially highjack the network? And if one entity couldn't take full control with 51%, what percentage would be necessary to cause serious problems, and would that be achievable?

If I might be allowed one more question, are these sorts of things non-issues on next gen coins using PoS and much higher total supply?