Rather the question is, would the USAF fly a B-52 airframe over 50 years?? The answer is yes.
Actually, commercial aviation put more stress than military aviation, simply because they logged more flight time per airframe with commercial plane than military fighter and airline have no problem flying Boeing airframe that been operated for a good 50 years. Airline only change or upgrade their airframe only either the aircraft is totalled, crashed or not profitable.
It is the same with military aviation, the only different is you don't change airframe that is simply "old" as long as it can still maintain, and it still get the edge, then they will still flies it. That's the eternal truth
Also, most F-16 airframe made during the 60s, 70s and 80s are already retired (scrapped, put into museum or storing in boneyard). Even if Romania is buying Blk 30 made in the 90s, they can still fly them at least til 2030s or even 2040 if they are careful with it. And by then still not many country have 5th Gen in their stock, even less with any meaningful number and F-16 would still give Romania the edge over majority of the country and suited their need.