Ethereum didn't fail to scale, its just taking much longer than anticipated. The plans are finally coming together though for PoS and Sharding on the base chain, with layer 2 there for more scaling as needed
> Bitcoin cost of 51 percent attack is ~400k for 1 hour. ETH is 90k for 1 hour.
Seems not wildly different to me and nearly the same order of magnitude, and you are also admitting that 51% is a concern for Bitcoin, so it sounds like we're agreeing now!
> POS uses few users to validate transactions.
The "few" can be quite a large number, and those same validators are by definition very invested.
