But it's not a software, it's a service that depends on a server farm, and a constantly evolving set of hundreds of third-party integrations.

It literally will break if left unmaintained if not funded properly, unlike a traditional software.

I do agree with other subscription-based models that sells software like Adobe Creative Cloud, where I'd be perfectly okay with having a non-updating version for a fixed cost.

If IFTTT was software that ran on your own hardware, I could see your argument. But they are not.