Do you think validators can easily manipulate validation process ?
Yes, for example by signing every version. Making a signature is cheap, while mining an alternative chain is hard. Which means, that if you want to trigger for example deep chain reorganization, then all you need in PoS is just re-signing the whole chain (which is a fast process, and you can easily reorg thousands of blocks). On the other hand, re-mining the chain is very hard, and some historical violation of rules are "grandfathered in", so they are written as exceptions in the code, because nobody will re-mine thousands of blocks. For example: BIP-30:
https://github.com/bitcoin/bips/blob/master/bip-0030.mediawiki // This rule was originally applied all blocks whose timestamp was after March 15, 2012, 0:00 UTC.
// Now that the whole chain is irreversibly beyond that time it is applied to all blocks except the
// two in the chain that violate it. This prevents exploiting the issue against nodes in their
// initial block download.
bool fEnforceBIP30 = !((pindex->nHeight==91842 && pindex->GetBlockHash() == uint256("0x00000000000a4d0a398161ffc163c503763b1f4360639393e0e4c8e300e0caec")) ||
(pindex->nHeight==91880 && pindex->GetBlockHash() == uint256("0x00000000000743f190a18c5577a3c2d2a1f610ae9601ac046a38084ccb7cd721")));
Do we think most Blockchain adopted PoS inorder to avoid computational cost?
I think they adopted it, because they still don't know, how to make Proof of Work chain correctly. One example of a failed attempt is NameCoin, which accepted only NameCoin-based block hashes, when calculating their difficulty. And because of that mistake, you can 51% attack NameCoin, even if you don't have enough power to 51% attack Bitcoin. What they should do, is to trace every double SHA-256 header, no matter if it is valid or not, and calculate their difficulty, based on that. And if they wanted to stick with 10 minutes per block, then they could just decrease their amount in the coinbase transaction, relatively to their local difficulty (or even not introduce any new coins at all, and just do a 1:1 peg with existing coins, and reward miners with their own product, called "domain names").
is this the reason why we still have lots of miners even if it requires joining a mining pool?.
Usually, you have to join a pool, no matter if it is Proof of Work, or Proof of Stake. Because there is a single chain of valid block headers, and you don't have "1000 valid blocks, at height 123,456". Of course, in chains like ETH, you have features like "uncle blocks", but then, that sword has both ends, and when you push that pool-related information on-chain, then validating everything from scratch takes more time, than it should.
Solving the problem of pools would require sending "shares" in a purely P2P way, so that each node would act like a new mining pool, and would share the block reward with every other peer, which submitted enough shares.
Miners And Validators - What comes to your mind?
I think those two functions are now somewhat separated in the Bitcoin network. You have ASIC miners, which just take 80-byte block headers, and produce a small hash. And you have validators, which are just full nodes, checking if the result of that mining is valid or not. In the old times, we had only "nodes", without splitting them into those who mined, and those who didn't mine. Now, the situation is different, but it was planned upfront to end in that way. Of course, there is still some room for improvements, for example some miners could just run LN nodes, and receive millisatoshis on L2, if they have too small hashrate, to compete in the mainnet.
