From Bitcoin's point of view, XT's "big blocks" don't exist, so the question of whether or not it can orphan them is moot. Core will continue to work on the longest valid chain, and that doesn't include any chain with blocks greater than 1MB in it.
That is not "Bitcoin's point of view". Please admit that there will be two (or more) implementations of bitcoin, each intending to be "the" Bitcoin. It is not objective or technically productive to decide from the start that the Core version is the right one, and the others are altcoins, no matter how much support each version gets.
Besides, the precise software that miners are running, what formulas they user, how they trigger the changes, what is their schedule for the following year -- all that is irrelevant. The only thing that matters is how many miners accept blocks greater than 1 MB, and how many reject them. So, instead of "XT" and "Core", it is better to say "big-blockian" and "small-blockian".
(Let's assume that there are only two size limits competing, say 8 MB and 1 MB. Things are already complicated enough in that case. If there are three or more candidates for the limit, then it matters how many miners choose each candidate.)
The longest valid chain accepted by Core *is* the longest valid chain, by definition.
Of course not; that is the longest valid chain *only for Core*. For implementations that accept bigger blocks, bigger blocks are (of course) valid, so the longest valid chain may be a different one.
Miners won't mine Mike's coin once they find they can't cover their expenses by selling them.
Just as they won't mine "Adam's coin" once they find that no one wants it any more.
Litecoin forked from Bitcoin.
Er, um, a mathematician would agree, I guess. Their blockchains share zero blocks, and zero is a number, too.
If, at that moment, more than 50% of the miners accept big blocks, the big-block branch (mined by them) will eventually grow faster than the small-block branch (mined by the rest of the miners). The split will be permanent and the big-block branch will grow faster.
Why would the split be permanent? Miners are free to switch between chains at any point, and will mine whichever chain gives them the best payouts. If XTcoin is valued significantly below Bitcoin then miners will switch back to mining Bitcoin and the Bitcoin chain will catch and outgrow the XT chain.
In theory perhaps the two coins could survive with separate identites. In practice, that will be extremely unlikely.
It will not be easy to use the two chains independently, because all transactions are in principle executed on both chains. (That is another reason why they cannot be called "altcoins".) Only transactions that spend coins mined after the fork will be executed exclusively on the same branch. So it will be difficult to find buyers and exchanges for each coin separately. Rather, just as miners will want to have a common size policy, the exchanges will want to accept the winning version only.
Have you read BIP101? It proposes jumping the blocksize limit to 8 MB next year, and then doubling of the blocksize limit every 2 years, for 20 years. That gives us 8192 MB blocks in 21 years. And that's what the sane bitcoiners should want?
Sigh, it wlll NOT give 8 MB blocks next year, nor 8 GB blocks 21 years from now. Those are block size LIMITS. Bitcoin had a 32 MB block size LIMIT for almost 2 years at the beginnig, but teeny weeny block SIZES. In fact it had a 1 MB size LIMIT since then, and yet the average block SIZE is still 0.45 MB.
It is sickening to see the small-blockians invariably replace LIMIT by SIZE in their arguments. Who is being disingenuous?
If the block SIZEs ever get close to 8 GB, it means that the traffic is 15'000 times what it is today, which ma mean a user base of hundreds of millions. Then of course the block size LIMIT would HAVE to be greater than 8 GB.
Anyway, 21 years is a long time. Who would have imagined the Chinese mining farms, a mere 6 years ago?
But I do agree that BIP101's schedule is stupid: it is silly to plan parameter changes several years in advance, when no one knows even whether bitcoin will survive to next year. The block size limit should be raised to 8 MB (say) as soon as practical, but without any automatic increases thereafter. If an when the blocks start to get more than 30% full (say) the limit should be raised again by another hard fork, to the appropriate size. Hopefully there will not be opposition, that time.
