You make a good point.
These planes are not inexpensive to fly, they guzzle fuel and maintenance has to to a big cost.
I spend years helping maintain them with the confederate air force. Cruse speeds for the P-47 probably uses 40 gallons/hr($6.00 per) and dry sump oil tanks hold gallons of oil, not quarts. FiFi, the B-29 has four of them turning. So it probably cost 1500 per hour for her, including maintenance.
Parts are almost impossible to find and engines are all decades old now and have to be rebuilt as new ones are not available. They can only be rebuilt so many times before it is useless.
Boeing helped rebuild FiFi years ago when she had to be grounded when the fuel cells in the wings started leaking. Not available but Boeing had her flow to Seattle and they, at their cost, had new fuel cells manufactured as well they had a bunch of old retired line personel do ton of work on her. I really believe before long the old birds will finally have to be parked for good.