Nothing. People do that all the time - there is a huge community for aftermarket custom ROMs at places like xda-developers. However, I would guess that is like less than 1% of users since custom ROMs a) are not popular b) require some courage and technical knowledge c) often void the official warranty.
D) Are highly unlikely to support relocking for new keys in secure boot, or are limited to a very small number of models.
The attacks sold to governments fighting such evils as journalists and pro-democracy youth haven't yet been shown to include persistency across reboots, AFAIK, but relatively small percentages of politically active people using insecure boot could encourage them to build those features to more passively monitor a sub-sample as it is relatively easy and makes their work less detectable.
