@croakerfish already hit the nail on the head but you are suggesting we buy a niche platform to do a niche job for an undetermined amount of time. As I mentioned earlier in this thread, using very expensive munitions to destroy low-cost weapons is an issue that the Navy (and DoD) are aware of and trying to mitigate but buying a whole new 'boutique' weapons platform just to do that is...not smart, or cost-efficient.
You point out that the USAF has a few unique airframes for unique jobs but some of those arguably cost more than they are worth. I remember a few years ago that the USAF CSAF pointed out that the funding for the A-10 fleet could fund 3 times as many F-16's, based largely on economies of scale since the A-10 has a unique airframe, unique engines (to the DoD) and all the other attendant infrastructure needed to support all of that unique stuff. And for what? Brrrtttttt? Frankly it has been Congress support that has kept them alive this long. Those extra costs are the same reason the KC-10 was retired even though it is 2 decades younger than the KC-135 and had more carrying capacity, a unique airframe with unique engines and all the attendant support for a relatively small fleet wasn't worth keeping them around any more.
Finally it comes down to capability. While on paper a light attack prop plane may stack up fine against a cheap drone you first have to get it in the right place at the right time to do so. Where are you going to base them? How long are you going to keep them there? In this case they are also a literal moving target without a fixed target, so you have to cover a
very wide area. What is the best platform to do so? A fast, long-range asset that can stay on station for an extended length of time and has long-range, all-weather intercept capability with a full suite of weapons to destroy them. Guess what fits that bill? Our current inventory of air-refuelable fighters already stationed in the area. Guess what doesn't? The AT-6, A-29 and OV-10 among others.
Ironic or not we already have very capable, 'low-cost' platforms to shoot down enemy drones, helicopters. The problem is again getting them in the right place at the right time to do so, it is easy to do so with a place like DC with HH-65's but it is a lot harder to do so with ships transiting within a hundred miles north and south of the BAM.