If you’re not doing every dungeon at +10, why you even need, or deserve gear higher ilevel than from a 15?
It’s as easy to play an alt in 9.1 as it was in 9.0. Even easier, because of gear inflation, dungeon and affix nerfs, and Tormented being considerably more forgiving than Prideful. The score also lets you mix and match dungeons, doesn’t require any dungeon timed.
I have 4 characters at 220+ ilevel, two I abandoned (not for gearing reasons, but because I didn’t like the class/role/spec/whatever), two I actively play. I play about half as much as I did in BfA, yet, have better gear and more fun.
You can very well play alts. You can even maintain multiple mains. All you need to do is set priorities and goals. For example, on an alt that does a weekly +5 or so, I don’t care about valor at all, because why would I? A potential reroll? No biggie, I will focus on that, and spam m+ with friends for gear, score, and Valor. I might run less keys on other chars to find the time, but that’s a trade I’m willing to make if I want to reroll.
The M+ rating stuff is nice, because it gives you more flexibility, you no longer need to time anything, and you can make up for a low sanguine depths with a high mists key for example, and that is amazingly useful.
It does take a bit more time, yes. But if you’re actively playing the alt, it won’t be noticable. If you’re not actively playing it, then it doesn’t matter.