On the consumer side, iMovie was probably the worst, not even being able to read projects made in the prior version. Not just the interface but the whole paradigm was different, only supporting a few features of the original program. I gave up on it.
iPhotos was another example: it had languished, but what came out had less functionality (no more external streams, for example) and simply threw info away (tags, face recognition, added metadata like location was all gone). Links to external tools were no longer available, though added later.
This happened to Pages and Numbers (& Keynote?) as well but I don't know if they count as consumer or pro apps. Consumer I guess.
On the Pro Side, replacing Final Cut Pro with Final Cut X (~2010 or 2011) pretty much drive video production off the Apple platform onto Windows. Like the iMovie case (I guess they didn't learn) it was a completely new interface without most of the features of the old version. I hear a lot of the functionality made it back in over the following decade, but you wonder why.
I think the loss of the pro production side in video (and replacement by Adobe) led to the abandonment of Aperture too. Apple announced that the consumer product Apple Photos was supposed to be the replacement -- what a joke!