Part of the attraction and frustration of blockchains has been their obedience to the laws of mathematics.
The answer to the stage of a blockchain you mention would seem to be, "If the blockchain isn't distributed and scaled enough to be resistant to a 51% attack, is there really any value remaining in it?"
If 51% attacks begin emerging, the chain still functions. People just need to require more confirmations. Then you require so many confirmations that the 51% attacks aren't profitable any more.
This kind of chain might become very slow, but still have the same properties as blockchains do have.
Btw the same thing can be thought has having a block time of 1 min, where de facto standard is awaiting 10 confirmations, versus just having 10 minute blocks.
At the point where 51% attacks are happening, it's fair that coinholders lose value. The alternative is that they are able to offload their bags onto some other greater fool who will lose the value instead.
