I think any of the alts could have made block sizes 100 times bigger at any point so unless all really have little understanding of the code, the argument that alts are merely waiting on software and that they would use it seems to ring hallow.
Most of them have probably accepted the code as is, because the block size limit is a long way off for any new coin. For an established coin the constant can't just be increased, the software needs a controlled switch-over. Doing it via majority block version number is best, although small communities can fork their code at a future block number.
The problem with the limit all along is that, for years, its effect is just psychological, until suddenly volumes ramp into the limit and it becomes a very real physical constraint. From a user perspective everything starts grinding to a halt, people can't transact without massive delays, they get angry, the press pours scorn on the failure, and says it proves a decentralized community can't be relied upon to maintain a currency. Some businesses have to look for alternatives.
In this climate all the alts which have a larger block limit trumpet their capability. One or two of them could gain traction and erode Bitcoin's first mover status. Serious erosion would result if nothing gets done.
I am unconvinced that sidechains help because sidechain volume also needs to be handled. Unless the SC is centralized, which raises all the trust issues again.