Not a bad solution and clearly the military is heading in this direction as well, though mainly out of necessity to preserve the wings of 5th gen aircraft. That being said, We still need to tain pilots in times of war particulalry since our wars are lasting 20 years a pop. And we currently buy A-29’s for foreign governments (Afghanistan)… which is why they have an aiforce designation in the first place. The real reason the A-29 makes sense, I would argue, in addition to an armed modern trainer (which has its own merrits), is really three fold…
First, they require a much smaller footprint in terms of maintainers and basing than an armed trainer as well much lower technical skill to maintain… Armed trainers are still jets and require decent runways, access to plentiful fuel, and a reasonably large team of maintainers to keep the squadron flying, etc. Granted, as you mentioned, the maintainers are already paid for; However, to put the A-29 purchase in perspective you can purchase an entire squadron of 6 A-29’s as well as the pilots and maintainers for a much lower price than it would take to purchase and operate a single F-35, to quote Camrade Stalin, “Quantity has a quality all of its own”. Also, a smaller team to deploy means you can actually deploy to more places that don’t have impressive infrastructure: think places in South America, West Africa or places in the middle east where our only other option to bring force to bear would be parking a Carrier Battle Group off the cost (a ridiculously expensive proposition for a limited counter insurgency for instance) or to fly the wings off of an hypothetical armed trainer based farther away, perhaps even in a different country.
Secondly, the A-29 makes sense because they are cheap enough to leave behind for foreign militaries without effecting our own military readiness and they are cheap enough for poor foreign militaries to maintain on their own after we leave. Think of an A-29 purchase as a training program coupled with a modern day lend lease.
And finally, three, they are a low profile asset… which is a corollary to point one… meaning it isn’t always wise to send a carrier battle group, or even a next generation armed trainer to certain conflict zones, even if there are approriate basing options for more advanced planes, particularly if we aren’t overly welcomed… Think Iraq In 2014, Seal raids in Somalia or countries in close proximity to major powers. As a hypothetical, consider a case where we might want to send a squadron to say the Philipines or Indonesia to help to fight off a counter insurgency or terrorist group while sending a clear message both to that gov’t and the regional power that we are only there to help out in one particular domain not overthrough the country or change the regional power balance with say China. Just a hypothetical, but you get the idea.