Ethereum started researching with pos in 2014 and they are still trying to implement it. It is very difficult.
By the way, POS causes more insecurity, so you have to remedy this. Also, aside from designing a consensus protocol, you have to design a network protocol for POS.
Defending your protocol from attacks in POW is very simple, but in POS it is very complicated, which can lead to hardforks, for example, as happened with Ethereum classic and Ethereum due to a security problem. So
maybe you think that you have it and it crashes, and you wonder how I fix it.
In addition to this, you still need a way to scale.
What Ethereum is trying to do, Cardano has been working for years, writing papers and trying to get this done. And they are succeeding.
By the way Ethereum use Account model, so it is more difficult
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I've seen recommendations from other guys in this post, I wouldn't invest in Solana