Forking on a whim, so some VCs who bought $1000 BTCs are that much closer to realizing their dream of everyone using BTC at Coke machines and Starbucks, harms the store-of-value function by diluting the previously more scare and extremely valuable commodity of blockchain real estate.
I feel this brings us to the crux of the matter. Ultimately, the anti-fork argument boils down to "Bitcoin works for us, and does what we want it to, so we should burn the bridge behind us and protect our interests. Let's stop here".
The pro-fork argument says "actually, we're not done yet, let's get some more people (directly) involved with this and see what further innovations we can come up with. We see a bright future ahead and it will be a change for the better".
And the best part is, you get to choose. There is absolutely no doubt in my mind that the fork has enough momentum behind it to go ahead. And when it does, you can stay behind on the old chain and stagnate in a niche with your precious limits, or you can stay with the clear majority and see where things go from here when enough people are on board to really challenge the widely accepted perceptions about how money should work.
If sidechains do eventually come to fruition then it's a win-win either way, but until then, the rest of us don't want to risk the network grinding to a halt because too many people wanted to try out Bitcoin and we hit a wall on some inane limit. The future potential for sidechains is not a substantial reason to delay this necessary update.
The simple fact is, you can't promise sidechains will be ready in time before we start piling up unconfirmed transactions.