In case of PoW the security adds up when a block gets older, in case of PoS this not the case

'Nothing at stake' and 'Long range attacks' are addressed with the concepts of bonded staking and finality.

Also PoS makes the rich richer, what leads to centralization over the long term

51%+ of bitcoin hashrate is currently controlled by three mining pools. The only way for POS systems to be more centralized than this is for only 1 or 2 individual stakeholders or staking pools to control more than 51% of stake.

PoS still have to be proven at scale and I think it will fail.

POS has been in existence for years and has yet to see a catastrophic failure, although the newest and most advanced protocols, casper and ouroboros, have yet to be thoroughly tested in production at scale.

I think finally there will only be one huge PoW chain and all the other chains will use the security of this chain.

This is possible. POS chains could also anchor to a POW chain for added security. It remains to be seen how this will evolve.

I know I will not make myself popular when I say this, but I think even Steemit will finally fail because of cartels of wales in the PoS system.

This is one of the biggest problems with steem, but I don't think it will cause complete failure. It may, however, discourage usage and investment if the problem is not addressed in a meaningful way.