Yeah, there was a lot of discussion about wing replacement going on after that C-130 crash, and there was an effort put in place to fund research into the feasibility of doing it. Most of that came to an end when the Brits jumped the gun and ordered replacement wing sets for their Nimrod fleet. They took measurements off an aircraft and ordered wing sets for the entire fleet, and what they found was although the planes were supposed to all be identical, there were differences between each aircraft that resulted from the manufacturing process and stress put on the fuselage over its service life and they ended up with a bunch of wing sets that were scrap because they couldn't be mated to the aircraft. The takeaway from that experience was if you have to rewing, you have to custom make each wing set for each aircraft, and that's prohibitively expensive.
The short-term fix in the early 2000s in the case of the P-3Cs and A-10s was to take planes from reserve squadrons and move them to active duty squadrons. Back in the 1980s, a decision was made by Reagan's DoD to upgrade active duty and reserve squadrons at the same time. It used to be reserve squadrons got planes that were retired from active duty squadrons, but Reagan wanted active duty and reserve outfits to be equally capable, so the reserves had planes that were as new or newer than some active duty units but had far less flight hours on them. A lot of reserve planes were moved to active duty squadrons and the high-hour planes from active duty squadrons were sent to Davis-Monthan for disposal. But now those lower-hour birds are getting used up and that's why the F-35 and P-8 programs have been pushed as hard as they have been because there's no more stop-gap tricks to pull out of our hats.
As for costs, there's a huge difference in flight hour costs between keeping a bird made in the 60s/70s flying as compared to keeping a new bird in the air. A lot of vendors for the A-10 are gone or have moved on to other products, which means a lot of small lot orders for spares and those are expensive. Plus most everything on planes of this era was mechanical or electro-mechanical, and not digital like today's stuff, so they are hard to make and expensive. Also, maintenance is much more expensive as trouble-shooting, repair/replacement, and line swaps take a lot of time. With the F-35, the aircraft transmits it's maintenance status to maintainers while it's on approach and informs them of any issues, and repairing them is a simple matter of swapping out a box on the flight line and the box gets sent back to Northrup for repair. I don't know what the per hour costs for the A-10 are, but I know they're about $18k/hour for the F/A-18 Hornets and they're much newer aircraft than the A-10s, and the per hour for the F-35 is expected to be about $10k/hour, and that extra $8k+/hr adds up quick for an operational aircraft series.
There's no doubt the A-10 is better at the close air support mission than the F-35, just like I would make the argument the P-3C is better than the P-8A for that mission, but the costs and risks of operating these old aircraft is just too high.