Sidechains will have to subsidize Bitcoin to a well functioning state else they cannot themselves function.  Indeed, there will probably be a competition among sidechains to subsidize Bitcoin (which has both pros and cons.)

This would hold as long as Bitcoin is seen as a credible and performant reserve currency, and driving it into reliance on and control by mainstream corporate infrastructure providers through excessive growth is the main thing which could cause Bitcoin to fail in this respect.

The magic here is that it gives Bitcoin a very credible means of survival even if native profitability fails.  This is key because there are some real question marks about how Bitcoin can achieve this on it's own given the sha256 capacity and distribution.