So now that we can design and build tanks with "obsolete" weapons it would be optimal to do the same with defensive structures, I don't think can be done without making the build menu massive by end game because we can't design structures.
So as a stop gap we should find structures that don't actually become obsolete when their replacement comes around, I'm not sure if obsolesce is listed anywhere on the site so we will just have to go by memory.
So far I find the worst case of structure obsolesce is the light cannon bunker which gets obsoleted by the hyper velocity cannon emplacement.
The HVC emplacement costs almost double the LC bunker (200vs125) while having roughly half the health (1330vs760) and armor (41vs20), It's also a HARD structure which has worse/different weapon damage modifiers.
If the Inferno flamers don't get obsoleted by the Plasmite flamer it should because they cost exactly the same.