a limit will be placed on the total value that can be transacted per block
As in "magically"? or by a hardfork? The context in which you're answering the question isn't Bitcoin anymore, it's a fork of Bitcoin.
Hardfork, of course.
Bitcoin is whatever people say it is. Bitcoin has had hard forks but we still call it Bitcoin. And Bitcoin clearly will have more hard forks in the future.
There is no "really needed" hashrate threshold that you can compare a subjective appreciation to. The question is flawed in itself.
Really needed hashrate = The hashrate below which Bitcoin will be a victim of a >50% attack. We don't know what it is, but it still exists.
No. Without massive protocol changes, the market does not have the tools to adapt to this situation.
That's your opinion. Mine is that miners will decide what is theirs to decide. If a group of "core developers" or the "foundation" wishes to hardfork its their problem.
Currently there is a limit of 1MB per block. Do you think this will remain forever?
If so, good luck using your outdated version of Bitcoin that cannot scale above 3 transactions per second. (You can use off-chain payments and whatnot but 3 tx/s is still not enough).
If not, then a hardfork will be needed, refuting your "Anything with a hardfork is not Bitcoin!" attitude, but more importantly - what will it be replaced by?
If nothing at all, good luck using your outdated version of Bitcoin that will have nobody hashing for it and be a constant target for hashrate attacks.
If it will be replaced by something... Then that "something" is exactly what we need to figure out.