There can't be a single cryptocurrency that has the "best design". It comes down to both the purpose and design. Yes, Bitcoin can be improved and is constantly being improved (e.g. Segwit).
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Bitcoin could potentially switch to PoS, but PoS is not the answer. PoS is not as good as PoW.
I agree with the first part. I like the actual competition between various cryptocurrency models, so there can be experimented with security and reward mechanisms. And yes, Bitcoin is moving forward (These "altcoiners" that think that Bitcoin is "outdated" have no clue).
But the energy problem potentially is a severe flaw. I've read an analysis that with a really widely-used Bitcoin the energy requirements should be as high as that of Finland. That's a lot (although I doubt the current banking/financial system consumes less).
The problem with switching to PoS is that there would be much resistance, as it would touch a key part of the protocol. So I don't expect it to be a move without losses. A switch to PoS would be much more controversial than the XT/Classic/Core discussion we had recently.
I think for now it's too early to say that PoS is "weaker" than PoW. I know the theory behind this statement (Nothing-at-Stake, "closed" system ...) but also the criticism of this theory (Vitalik Buterin's Weak Subjectivity statements). And no successful attack on a larger PoS coin has been carried out until to date. That may be related to the small market cap of actual PoS currencies. So I'm curious what will happen if Ethereum switches to PoS.
And there may be other systems we don't know for now and that are better than PoS.