If there is one fork with 5-10% of the hashing power why do you think anyone would care about it?
What do you mean by dump their coins on the other fork? You mean like try to sell them on an exchange? But the majority of users would be on the new fork right so they would have the majority of coins so why does it matter if the core developers dump their coins? Do you believe it to really be so fragile that a dozen users could crash the whole thing just by selling their coins?
Have you considered that the real reason there hasn't been a change is that because the people that want it aren't actually in the majority?
Are you actually for real?
This is the current mining pool distribution: https://www.blocktrail.com/BTC/pools
Antpool, which represents 17% is in favor of 2MB, and has threatened to NOT include any Core changes until it happens.
Proof: https://www.reddit.com/r/Bitcoin/comments/4l564f/bitcoin_cor...
f2Pool, which is 23%, supports 2mb.
Proof: https://bitcointalk.org/index.php?topic=700411.msg13571787#m...
BTCC, which is 13%, supports 2MB
Proof: https://www.reddit.com/r/Bitcoin/comments/40az4o/bobby_lee_b...
BW, which is 12%, supports 8MB! Huobi, (HaoBTC), is 7%, and also supports 8MB.
Proof for both: https://www.reddit.com/r/Bitcoin/comments/3a5qj5/draft_signe...
KNCMiner, which is 2%, supports bitcoin classic (2MB hardfork)
Half of slush pool (5% total, so 2.5%) supports classic.
The only major miner that hasn't explicitly supported some sort of block size increase, is I believe Bitfury, which is ~10%.
Now, please add up all of these numbers, and see if it adds up to a majority.
My other points that you skipped over all still stand. If the majority truely wanted change they could do it painlessly. But they don't so the minority that does has to find something to blame besides lack of support so you come up with a ridiculous dream in which a handful of developers are holding everything up.
And if they do that, then there is a chance that there will be two competing forks, instead of one, and each competing fork will dump their coins on the other fork, and basically cause the whole thing to crash.
Core can't "win" by doing this, but they can certainly make the rest of us lose. This threat, along with their constant insistence that improvements are Just Around The Corner, has allowed them to delay efforts to coordinate and make the changes that the community wants.
Eventually people will wizen up to the fact that their improvements are never coming, but until then, they are playing the delaying game to buy themselves as much time as possible until they are able to get their competing, lighting network out of the door.