as for sticking to the oldies but goodies there's a few examples I could mention like the B52 (which has now been in active service for more than half the time we have powered flight as a species

),or USS Missouri and USS New Jersey and let's not forget they were still using t-34's in the Balkans in the early 1990s
so yes, upgrading the type VII and IX would probably have been better (same goes for pzkfw. IV and VI) but I wouldn't want to have been the officer commanding the British Taskforce when 1 of those type XXIs surfaced right in the middle of them to surrender at the end of the war and they didn't know it was there

, that definitely puts a dent in your promotion prospects.