Very interesting observations, and an interesting topic.
I think the number one thing that "rulings not rules" does it put the burden for more things on the DM's shoulders rather than on the rules system. With a good DM who listens to their players and take their feedback into account, that can be a great thing.
The problem is that the quality of DMs runs like a bell curve, and so you have half of your DMs in the average to poor end of the spectrum. For those DMs, if they're not going to go through the process to be better, rulings can seem arbitrary and contrary to what makes a good game.
I've seen this first hand recently, as I'm playing in Organized Play and the DMs I've been with have been terrible. The thing is, these are the same people who ran decent games under previous editions because they just stuck to the rules as much as possible. They're gaming without much of a net now. I hear "no, you can't do that," a lot, and I'm a "go with the flow" sort of player more than anything else.
I've heard a lot of great stories about 5E from DMs here on ENWorld, and the common thread to all of them is that they've seemed like awesome DMs for as long as I've been reading about their games. Heck, some of them have been great DMs for decades. With a good DM you can play any game system and have a good time (a particular DM I play with could run a Chutes and Ladders style RPG and I'd show up) but, to be honest, if DM quality follows a bell curve, which I believe it does, you've got about half of them who just aren't that great.
I think the trick is to do everything to turn a poor to average DM into a good one, and the best way to do that is to encourage them with good suggestions in the rules themselves, and most importantly in any adventures they might be running.