The B52 is a very good example of why a polity might not build at the latest TL. Firstly, the old airframes are still good, so no need to replace them. Secondly, while a much better 'B52' could be build today, the cost would be enormous and the old ones still do the job.Only because there is no higher TL item that is better, and modern artillery barrels are much higher tolerance than WWII era ones.
A10s and B52s are not WWII technology.
One thing that doesn't seem to have happened is ripping out the electronics and upgrading them, or the power plant. One possible reason is that most electronics in CT/MT don't improve much between TL-14 and TL-15 (some comms and a few sensors not usually used in combat vehicles get smaller - that's about it) and those that do get bigger (but replacing the power plant would make plenty of room).
But here's the thing - the Trepida grav tank was adopted by the Imperials in 1109, and the Astrin a few years later, both long after the Imperium officially reached TL-15. The Imperium made a concious choice to use TL-14 technology for their primary 'ground' combat vehicles. Presumably having a wider ranges of worlds they could buy from was considered more important than having the very best, at least by however got to make the final procurement decisions.