> a couple thousand maximum
Maximum? Many pups cost more than this and you'd be paying to replace at least. You might also be paying for any amount of vet bills if the animal isn't killed on the spot. If you hit a service or working dog, be ready to shell out tens of thousands for replacement due to the unique tasks performed (not all dogs can do them) and specialized training they receive.
In any case, Wag and its walker are probably not at fault. I'd be after the driver.
Of course, I don't like the idea of a stranger walking my dog anyway. Even a kennel isn't ideal, but at least then I can build trust with a particular business. Wag just puts my dog in the hands of random people I'll probably never see again, opening me up to liability and my dog up to risk.