I think one of the most important considerations here is the fact that in Star Trek, the warp core trumps nearly everything else in terms of importance. Having a better warp core doesn't just mean a faster ship, it means stronger shields, it means harder hitting weapons. It's more power for life support, more power for your auxiliary systems ,for your computing systems, for your sensors; everything. incremental improvement in other design areas is okay, but revolutionary improvements in warp core technology make the entire fleet massively superior without changing anything else, because EVERYTHING draws from the warp core.