I think you have quite a while (years). I bought an 11 Pro Max in February 2021. That's an LTE phone, no 5G. I'd be pretty upset if my carrier cut off LTE soon. In other words, LTE phones are still being sold.
3G is only being shut down just now. T-Mobile is shutting it off and Verizon is doing it at the end of the year I think. How long was 3G around? How many 3G only phones are still out there?
As far as #2, well, maybe. Most people are on LTE now. Once more of them shift to 5G that'll probably happen. But then your time on LTE will start to be less and less because most people will have moved on to 5G.