They're iterating to find what works. Far better than letting problems continue to run on.
The most promising thing to come out of this for me is the now fluid nature of mechanics and planes. They aren't tied to burning through mechanics too quickly, nor are they required to stick with those with shallow design space. Similarly, not every story will be "set up plane > world ending crisis" anymore; they can do smaller scale stories- or larger- it doesn't matter anymore.
Cautiously optimistic regarding most of these changes.
Did I miss rotation talk in there anywhere? How will it work now? One large set rotating out every year or 2?
Man, they'd only burn thru mechanics faster if they added sets. They never did. They're going through them slower now that there's less sets.
What? There are 4 expert level sets with one off mechanics now; not 3. Additionally, mechanics tend to be tied to blocks, not individual sets, so the rate at which they burn through them has increased as they've found themselves leaving planes and their associated mechanics- like Kaladesh and energy- more quickly.