Sinking submarines is just a bonus. The really important thing to do is to keep them from attacking, which blimps are quite effective at.
This is a completely circular argument. Aircraft can keep submarines submerged just as effectively as blimps, and can do many things blimps can't. Such as bombing, carry cargo, training, etc. etc. OTL the aircraft that really closed the gap were ordinary B-24 bombers with a few extra fuel tanks and some bits of specialist equipment. It's a lot easier to build and support a handful of extra bombers than a whole specialist branch with basically zero overlap.
Well...I can see why you would choose airships, actually:
- The POD is long before 1939, say 1929 or 1919. In that case, the competition is between airships and...well, nothing. Seaplanes, I guess. There weren't long range aircraft of the B-17/Stirling type at that time, so airships are your only real option for long-range/long-endurance aircraft. If you take the submarine threat seriously and decide to heavily invest in aircraft at that time, you're probably going to be building airships for that reason.
- It's a compromise between Bomber Command and Coastal Command (or the equivalent). B-17s/Stirlings/etc. can be used to bomb Germany, so the parts of the Air Force that want to bomb Germany will want all of those aircraft, rather than leaving some off for anti-submarine work. To the extent that airships don't (necessarily) use the same resources as heavy bombers, procuring those instead of heavy bombers for the latter role could help soothe those tensions and help both sections get more adequate resources.
On the whole, I feel the first point is more reasonable, but the second one isn't totally crazy, at least so far as I know.
Your first point is exactly what I was saying earlier. Moffat loved anything that floated in air, so the US navy ended up with a bunch of expensive toys that could do a poor job of ASW patrols and nothing else, hence half a million hours of blimp ASW patrols and by some estimates 10,000 people tied up 'keeping subs submerged' with zero possibility of damaging them, when they could have been keeping them submerged with at least some small chance of damaging them.
Your second point is to be honest nonsensical. USN blimps used radial Pratt & Whitney engines, the same radars, MAD and sonobuoys you would fit to aircraft, enough aluminum to build a gondola holding 10 crew, and the same skilled navigators and mechanics etc you would want in carrier aviation or bomber/fighter squadrons. No matter how you dress it up the blimp program is directly robbing resources needed by the Air Force, fleet air arm (and even heavier-than-air ASW patrol!) and someone high up is going to have to slice the pie and decide who gets what.
"You lose 5 bomber squadrons so they can build and operate some rather ineffective blimps" is in no shape fashion or form better than "you lose 5 bomber squadrons so they can use them to effectively prosecute ASW and convoy protection" and its positively criminal if it becomes "we can only get 5 ASV radars and 2 MAD per month and they are going on the blimps, the bombers can use the Mk1 eyeball".
Just for a comparative statistic of uselessness, in the bay of biscay in 1943 about 5500 hours were flown per month, with roughly 60 sightings, 30 attacks and a bit under one sinking per month. So if "good" looks like one sinking per ~6,000 hours on patrol, what do we make of those hundreds of thousands of hours with nothing to show for it? Unless we heroically delude ourselves that subs were far more likely to stay submerged under blimps that were a hundred times less dangerous to them than conventional aircraft, we can only conclude they were a wasteful diversion of effort. I have certainly never seen any evidence that U-boat crews had any special fear or awareness of blimps.
