As far as core rules being the default, isn't it more important to test the new rules in EA? The core rules have already been tested and a game mode for it can come later.
Having played some Solasta, I can see why they'd want to spice things up with new actions and environment interaction. Doing the same basic attack over and over gets a bit stale. Not to hate on Solasta either, I really like how they handled group dialogue, resting, and traveling on the campaign map. Both games have their pros and cons. I'll be happy playing both games as they come out for different reasons.
As I’ve said. Capturing the big picture is important to know the impact of the rules that they’ve changed. If you go through the pages of this very post you’ll see some. That’s not for the sake of “be faithful to raw DnD5e” this is more about balance. No one is complaining about weapon skills in the forum because they do not break the system. As is, the current rules destroyed spell values. And taking spells out of the core of the system will do no good for the joy of the players.