I am wary of sidechains because of the mining incentive issue. Moving transactions off-chain leaves miners underfunded in the far future, or else results in very high tx fees, leaving Bitcoin more open to competition from a more inflationary coin that pays miners through block rewards more and tx fees less.
However, on that note I have to admit there is little substantive difference between sufficiently high tx fees and slightly breaking the 21M coin limit (no halving, or slower halving), since you can't actually benefit from your Bitcoin wealth unless you spend it eventually, at which time if you incur a big tx fee as a tax it is no different to you than an inflation tax. OK, maybe certain people will gain/lose more from high tx fees vs. inflation. Nevertheless, one way or another the miners have to get paid, and if Bitcoin is severely under-mined due to SC then one of those two things will eventually happen, or else Bitcoin (the platform) will die.
I agree with the rest of your post but I have issues with this notion here.
From my point of view. SPVP Sidechains that enable merged-mining are in fact bringing balance to this issue of transaction fees emigrating to different, off-chain schemes.
So far the only option to accomodate transactions types that are not implementable in the mainchain had been these off-chain schemes that effectively present this very danger of moving transactions off-chain and out of reach of potentially underfunded miners.
SPVP Sidechains will not negate this aspect but they introduce an alternative for the market. The likely outcome is that any chain that gain significant adoption from the market and therefore require the "ultimate" security model will be picked up by miners and that way these transactions will not "escape" them.