Isn't this basically the case with github now? i.e all the cryptos have their code public on github so if I wanted to start mining, I'd have to go there to get the code
With proof-of-work chains, one can download the blockchain even without a client and write a simple program/script to calculate the total work (hashes) embedded in the chain. This provides a completely independent measure of trust.
With PoS, you must assume you can trust the client or root of the blockchain, which really kills the primary use-case of a decentralized currency.