It's quite possible to remove content from the blockchain. The blockchain is just code, and the witnesses are the ones that post it. Code is infinitely mutable, and if the witnesses agree to do it, blocks can be deleted.
It would only take 51% of them to agree. Less if the blockchain is severed physically, as in firewalled in half. Then it would take as few as 34%.
Too many ifs.
It's all ifs, all the way down.