It's not like ICE cars (both new and used) aren't still being sold in tremendous amounts in the US…

I think something like 4-5% of car sales in the US are electric-only or hybrid, the remaining 95% being old-fashioned ICE. It's not just Mexico still buying them.

I'm not sure if they were ever in use outside the US.

I definitely only meant it for the TSA, and I'm glad you pointed this out.

However, I suspect that in these countries that have the financial constraints you describe, opting out may not always lead to decommissioning them because installing alternatives requires significant expenditure (which is why the US could afford to get rid of them in the first place). I don't think this is universally true (i.e. I expect some countries ended up with old backscatter scanners but would choose to replace them if many people opt out) but I don't know that this intersection is very large.

there's no value add - other countries aren't really concerned with security theatre to the extent that the US (maybe UK?) is. witness the difference between screening at e.g. international airports outside of US/UK and then additional necessary screening for US bound flights.

perhaps, though, that makes it a great export opportunity there, similar to arms exports: US law will mandate that these scanners are minimally required for inbound flights, other airports pick them up in order not to lose US flight slots.