This is a good change, and they should have done it much sooner. I get not wanting to do that much errata right away (what if Planeswalkers had failed?), but once they decided they were going to be an every-set thing, rather than a once-in-a-while thing, they should have bit the bullet and made the changes.

This will be a mess.

Everybody tests online, and there the errata will be on the cards, so they will get an opportunity to play it the right way before hitting stone age play. My only concern is that some card text might get too long to fit and be a restriction on future designs.

Also, this opens up design space, as you they can now design burn that hits any combination of creatures/players/planeswalkers, while before, the burn that hit players automatically hit planeswalkers too.

I think that, at least in the near future, they'll have to have player burn always be able to target planeswalkers, because of the errata issue. Large-scale errata works if you give players the ability to easily translate from what an old card says to what it currently means. For example, that could mean "any targeted burn that can hit players can also hit planeswalkers". However, that also means that they can't print new cards that look they're just pre-errata old cards, since people would also apply the translation rule to those.

For a concrete example, if you had a 2015 Lightning Bolt and a 2018 "[this] deals 3 damage to target creature or player", you'd expect them to work the same way. If playing correctly relies on knowing when the rules change happened and whether a particular card was printed before or after that change, there are going to be a lot of misplays.

Yeah, as I said above, they're essentially closing off that design space rather than opening up, and I think for more than just the near future. Probably not forever, but for a good while.

Since the last block I've been wondering if WOTC was ramping up for a new legends-matter set. Maybe it was just a prelude to this, or maybe this is just a prelude to that. With Dominaria on the schedule I'm thinking the latter. This could have a lot of impact on legends-matter in constructed.
I like that the rules seem a little more streamlined now. I'm not a fan of the flavour implications though, and I feel like the pw uniqueness rule's design space has been wasted. I rather liked the idea of having to choose between different versions of the same walker. There are power level considerations as well, and I'm not optimistic even though I doubt it will be a huge deal.