Wouldn't it be the exact same centralization situation if the sidechains in question instead were developed as altcoins? The altcoins/sidecoins that use merge-mining may lead to centralization while the ones that do seperate mining probably won't?

Or is there any difference?

The problem is that bitcoin mining has very slim margins now, and if sidechains happen, "mining difficulty" will go even hight and mining sidechains will be pretty much be mandatory for ROI.

However, mining many sidechains requires significant computing resources.