Anything over £4.99 in the App Store I just skip past
Topaz is a revelation as you say, but still overpriced, but then I think pretty much everything photography related is overpriced these days
I think it's poor that Sony don't offer these features meaning some folk need apps like this, clearly the cameras are capable of it, it's just a matter of making the software.
I don't focus stack so it's not something I'd buy anyway, but I'd want to know that it's future proof i.e. it's not going to stop working if a camera has a firmware update.
EDIT: the demo on YouTube isn't great, doesn't show how the final image is achieved.