Right in the middle of heading back to my civilian job now. I always planned on going back, so maybe a little different than your situation, but you can always look at it as...what if you need that job for some reason?
Who knows what the world situation will be when you pop out at the end of the pipeline? Your unit may not have orders to keep you going as long as you plan; the airlines might not be hiring; you might find yourself without a viable option for a another civilian job; or (not trying to put the evil on you, Ricky Bobby, but gotta give a legit look at everything) you might just not make it through UPT, so you might need to go back to the civilian world.
Hell, even if all goes well, there can be some serious breaks in the ANG pilot training pipeline, from my understanding, and it can vary by unit whether they keep you paid/insured and on orders during them. You might find yourself waiting for a few months between OTS and UPT or UPT and PIQ to where you might need to have a civilian gig to keep a roof over your head and yourself fed.
So, why leave your USERRA protected job in the dust when there’s at least a (hopefully small) chance you’ll have to go back to it? The fact they pay you Mil leave is a nice positive kicker, but I’d honestly hang onto the gig more so because the world is uncertain and, whether you love your job or not, it is one you can have legally held for you while you journey through the pipeline.