The Rommel and Patton tanks were introed in 3027 when the IS was already starting to recover and the FWL still made a ICE version called the Axel in 3036 so fusion engines were becoming more available but not everywhere. So there would still be a need for alternative to standard fusion engines. Primitive fusion engines would be a good replacement but with fusion being so much better than the alternatives I suspect that if anyone tried to produce them they'd have been a target for nukes or ROM.
There older tanks that predate those two main battletech tanks such as the
Tiger Medium Tank was ICE, the
Behemoth Tank was still chugging along, could be easily be primitive-ized. The Estevez Main Battle Tank which was primitive tank to begin with could have been brought back as well with perhaps better gun, such as AC/5 or dare I say keep the Heavy Rifle it was sporting.