There is no concrete relationship between the number of users and the value implied by some portion of those users.
(Ridiculous thought experiment: One user controls 75% of bitcoin and will buy and sell them only for $5. Given enough dollars desiring bitcoin he would quickly be reduced to 0% control, but in the meantime everyone else would have some difficulty selling for much more than $5.)
You are right. What I mean is, if half of the coins will go with a forked blockchain, another half of the coins will not be able to participate on it. So for a guy who is about to receive a payment in a forked coin, the market would be roughly 2x less liquid. It's the same with altcoins: if I pay you in Litecoin, you'll get 11x less liquid object than BTC. Maybe it has the right price at the moment and is fine by you, but as it's less liquid, it's less certain to hold its value over the longer period of time.
Yeah, my objection was to the precision that your wording implied.
(realistically, I think the outcome of an unpatched fork is the collapse of bitcoin and people becoming very wary of anything claiming bitcoin in its genealogy (or even inviting close comparison to bitcoin))
No, accidental fork will be reverted and people will go on with the previously working software. Bitcoin can collapse only when it's non-recoverable (e.g. ECDSA or SHA256 is completely broken). If it is recoverable (like in all cases mentioned above), then there will be just a short period of turbulence.
