Bitcoin is an incentive model. As it states in the white paper, if a greedy attacker is able to assemble more hash power than all of the honest nodes, that attacker still has to choose between defrauding people or getting paid to validate transactions. This works because the protocol does not change. This is very specific. The incentive model of a blockchain does not allow protocol change.
Very simply, any time a new addition is made to the protocol, that splits the network. All of the old systems, all of the SPV nodes, all of the people using bitcoin continue on the original protocol. In telling people that they need to update their software so that they have the new protocol developers are misleading people into switching from bitcoin to another system. This is why the 51% attack is not a problem. Even if an attacker gains 51% of the network, the best they can do is spend their own transactions or others that they are involved with a second time. In doing this they have also left an immutable evidence trial. This is admissible in law.

CSW
Mar 6, 2019
https://metanet-icu.slack.com/archives/C5131HKFX/p1551866901779900?thread_ts=1551866901.779900&cid=C5131HKFX