> then my tests become integration test behemoths.

There are no consistent definitions found in the world of testing, but I assume integration here means entry into some kind of third-party system that you don't have immediate control over? That seems to be how it is most commonly used. And that's exactly one of the places you'd benefit from enabling multiple implementations, even if testing wasn't in the picture. There are many reasons why you don't want to couple your application to these integrations. The benefits found under test are a manifestation of the very same, not some unique situation.

Not really. Sometimes you just want to mock some bigger system that is still internal/local. And sometimes it is an external system, but it makes no sense to wrap some sdk in yet another layer, if you won't ever swap it out.