The only way to avoid that is if that $100 buys you actual ownership, like the ability to have your own secure boot keys and modify the software. So long as Apple still owns your phone, they can alter the deal, and there is nothing you can do about it.

Perhaps not even that is completely safe long term, as companies can introduce a locked down dependency, reverse policies (see Google's recent sideloading stance), or find some other workarounds.