That was almost 7 years ago, when Mecanim was first introduced. A Unity employee recognized almost 7 years ago that Mecanim’s workflow is too painful for some scenarios, yet the solution is still “use Legacy”. If you don’t want to/can’t use it, your only options are getting this paid asset, making your own solution, or using Playables (and I’m not even sure about this one). None of those are proper solutions for something that should be standard.
The only proper solution I can think of is to add an equivalent to the Animation component, removing the requirement for the extremely inflexible animation controllers. Or better yet: Try to find a way to make the current Animation component compatible with Mecanim while remaining backwards-compatible with Legacy animation clips.