I don't think your conclusion is correct. 4G -> 5G will result in 5G networks completely consuming the 4G ones. T-Mobile has already been talking about shutting down LTE in a few years. The reason afaik that 4G and 5G are seemingly compatible right now is that most networks are in 5G NSA mode (non standalone) which means that LTE is actually used for negotiation and orchestration stuff, with 5G only used for the actual data pipe. I have been told that most of the benefit of 5G comes with 5G SA (standalone), but that requires not supporting the LTE stuff

Ironic given that LTE, literally called Long-Term Evolution, was supposed to make future backwards incompatible changes unnecessary.