Like anything that exhibits strong network effects, a blockchain is a natural monopoly of sorts. Free market forces will always push towards the ONE big blockchain.
There is a strong incentive for new users to choose the social networking site that already has most users, to edit the wiki that already has the most content, and so on. This discussion about rebooting the blockchain reminds me of Wikipedia in the earlier days, when several Wikipedia clones popped up, all of which failed or remained extreme niche of course.
IMO network effects in Bitcoin are even stronger than in Facebook or Wikipedia. I think that is it inevitable that new chains will appear, but the older and longer the main chain becomes, the harder it will be for them to stand a fighting chance. Right now, I think that only Gavin has a chance of succeeding with this and soon not even he will.
The only exceptions are chains which fulfill a very different need than the main chain, such as Namecoin.
None of us can pretend to know if we are in a bubble or whether it will pop. There is no objective way of measuring this reliably. This is really nothing more than a hunch of yours (and other users). There was bubble talk after the first price spike in July 2010 and people were complaining that 0.06 USD/BTC was grossly overvalued
bubble, if it ever was a bubble, never popped of course because the goods and services offered for BTC on the internet eventually caught up with the speculators' expectations.
Is it really worth doing something as drastic as a chain reboot, based on a hunch? It wouldn't even solve the supposed problem, because the new chain would be just as prone to speculation as the old one.
I don't think speculation is such a bad thing anyhow. The market has already decided that Bitcoin is useful as an investment vehicle on top of being useful as a way to transmit money. The market isn't going to change its mind soon, and even if Bitcoin became primarily an investment vehicle, that doesn't mean the economy won't be sustainable.