It would be possible to fork to create a higher limit with that hashpower. It would also be possible to have 51% without making it publicly known. But nobody needs to bother with that when they can just double spend.
A fork can be made with any amount of hash power, it doesn't need to be the majority. A fork happening doesn't mean it will be followed, or even contentious.
51% attacks do not allow double spending on the blockchain. If a block is overwritten the attacker may choose to change the destination of their own transactions for that block, but the previous spends are undone. This is why merchants wait for multiple confirmations. Typically 6, which would require a 98% attack to undo.
At this point it'd take a nation-state to produce that much of bitcoin's hashpower, if it's even still possible.
