Bitcoin should never ever be forked like this unless there was a crisis. There is no imminent danger of hitting the block limit:
https://blockchain.info/charts/avg-block-sizeI bet 100 Bitcoins that if this Bitcoin XT fork was ever implemented it would cause huge problems and more forks would be needed to try and stabilize the blockchain. Worst case scenario is Bitcoin becomes unusable and loses all of its value.
Forking is a nuclear option when the blockchain is no longer functioning right, the only time a fork was needed was to fix a fork that Bitcoin QT developers accidentally created by releasing a faulty wallet.
Bitcoin is not like windows or an iphone, it doesn't need constant updates or any forks. If it's stable then it should never be changed again. The chart does tell us that a certain group tried to manipulate some numbers recently. However I do not believe that Bitcoin XT will cause "huge problems". It is essentially the same as Bitcoin Core but with just a few additional patches done by Gavin. I don't think that it will cause huge problems.
Forking is necessary. I'm not talking about the current situation. Your last part is incorrect. It should update whenever possible, including bugfixes and improvements. However we should only do forks if we reach consensus (not that easy).
Once again, using a fork to modify Bitcoin is a nuclear option. A fork has never intentionally been introduced into the blockchain.
It also ruins the decentralized nature of Bitcoin to have developers changing it with forks. No true Bitcoiner would support such a centralized action, and it sets a bad precedent for the future.
Undoubtedly if Bitcoin XT gained enough nodes to fork it would cause major losses of Bitcoin and destabilize the market. Anyone who stays on the fork that becomes shorter will be prone to losing Bitcoins. I think it's possible this could backfire severely on Bitcoin XT users, if the fork is implemented and then the community races to increase Bitcoin Core nodes/mining then there will be major Bitcoin losses for those on the XT fork.
Imagine the confusion if someone on Bitcoin XT after the fork sends Bitcoins to an exchange that's still on the Bitcoin Core fork. Their Bitcoins would never appear on the exchange and may be lost forever.
This is such a bad idea that it's possible Bitcoin XT is being created by a group that wants to destroy Bitcoin. It's like a crypto trojan horse, if the community lets it in thinking it's a gift then it will destroy Bitcoin from the inside.
Unfortunately the Bitcoin network is so weak at this point that it might be quite easy for XT to fork even if barely anyone supports this. Less than 6000 nodes and dropping, and mining power has leveled off at 350 petahash/second for the past 5 months. Since processing power becomes cheaper by the day it is becoming easier for someone to fork the network, any major corporation or the government could easily afford it. If mining power doesn't increase anymore then even a lone evil doer could successfully attack Bitcoin in the near future.
So much fear and panic for no reason. None of those scenarios you've described are particularly likely. There's only a slim chance that any exchange who knows what they're doing would willingly stay on the chain with less support. They would risk damaging their reputation if they lost their customers money as a result of following the wrong chain, so there's no incentive for them to do that. The only exception to that would be MPEX, as they've stated they won't accept any changes, but screw MP and all of his crackpot supporters.
The network hash rate has stabilised, but hasn't dropped by any significant amount. The network is in no way, shape or form "weak". In fact, it's more than twice as strong than it was this time last year. The fork will only go ahead with majority support, so what you're saying about the fork being easy even if no one supports it is simply nonsense.
If you think a fork "destroys centralisation", you clearly have no idea what the word means. The users securing the network decide what software they want to run. If the majority decides to support a change, that change goes ahead. Anyone, core dev or not, can release modified code, but it's down to the users whether or not they decide to support that modification. There's no logic in saying "one group of developers thinks A, so we all have to agree with that, but if you agree with this group of developers who thinks B, suddenly it's centralisation". You can't have it both ways. If you genuinely wanted a coin where a majority of users can't change the protocol, then you would actually want a centralised, closed-source coin where users don't have a say. If that's what you want, I'm afraid you're in the wrong place.
I honestly can't tell whether you're just giving into hysteria through lack of understanding, or if this is a deliberate attempt to FUD. Either way, most of what you've said needs some serious rethinking. For someone considering themselves a "true Bitcoiner", it doesn't seem like you've grasped certain core concepts very well.