if OP_CHECKDATASIG is beneficial btc can implement it

With Bitcoin's history of extreme hardfork aversion, it's not that simple. In theory you're right, but in practice it's much harder.

You should be able to add an optional functionality w/o a hardfork yes?

You can technically introduce anything as a softfork yes. The only difference is how difficult it is to implement, which I think would go beyond "just copy it".