[TangoTrotFox]

51% attacks are mostly overblown. If a group decides to engage in a 51% attack, all they are doing is effectively revoking their own license to print money - which is what having a > 51% mining capacity in a major crypto translates to. Bitcoin is a public ledger and double spend transactions would be completely visible to all. All this means is that the coin would end up getting forked, similar to what happened when Ethereum screwed up - and it continued on with minimal fanfare. And in that case their screw up was of their own fault instead of something inherent - even fewer people would be inclined to stay on a Bitcoin line that has been 51%'d.

Rolling with the mine analogy, a 51% attack is equivalent our group that sent in a million miners changing the diamond mine into a 'glass' one. It becomes fake, and loses nearly all of its value and people move to the new 'real' mine. You do nothing except critically hurt yourself.

[Retric]

This assumes the entity wants Bitcoin to survive. The Chinease government has zero intrest in keeping Bitcoin alive and would happily kill it off.

Forks don't help with 51% attacks as the attacker can continue to use their hardware on the fork. Further, you can use a proxy to hid the origin of a block, so Bitcoin would need to move to a new hash which would take a long time and prevent any obvious successor.

[TangoTrotFox]

I was making the assumption that the entity wants to kill the coin. Doing a 51% attack out of greed makes no sense as it'd be immediately self defeating, at least on a mainstay coin.

There are numerous different hashes with varying levels of ASIC 'resistance' - yeah it's a cat and mouse game, but hardly a major issue. The 'obvious' successor would be a matter of the unpredictable public as I'm certain numerous entities would vie to become e.g. Bitcoin 2.0. In the end the market would decide which was the winner. The great thing about it all being that the users could actually come out net winners in the end as the successors would likely not only be technical superior, but it would also be able to use the exact same ledger (as with Bitcoin cash for instance) so that you start with the same relative share in it as you did with the original.

In a way it would even be a good thing as Bitcoin itself is increasingly dated technologically, but is dominant because of its market positioning - which makes it difficult to change. If its market positioning was damaged, we could see the rise of an improved successor.

[Retric]

A few weeks/months/years of not being able to trade Bitcoins at old value is a real downside.