Look, just because you dislike players that do mythic difficulty/m+ over 15 doesn't mean they are your problem. The problem is the lack of content.
If you remove mythic and cap m+ at 15 you won't magically get more content, you simply lose some content and the players that go with it. And this is not expensive content, mythic keeps a mythic player playing for months on average, the same raid on normal will be cleared in a week or two by many players in the "ok" category, and they'll be done with their journey.
I'm not saying that the ok players are doing anything wrong, I'm simply saying you won't magically get the content you want and that casuals need from removing a couple of weeks of tuning from mythic raids.
What is needed is resources, more development time, more art assets, and interesting ideas for playmodes. Imagine for instance if we had a 10 man raid incoming from the PTR now, perhaps wth 7-15 people scaling, no hardmodes, somewhere just below heroic gear with some badges to collect to buy 3 or so 252 pieces from a vendor. I imagine that would be quite popular. Difficulty wise somewhere between just between LFR and normal for the first few easy bosses and between normal and Heroic for the last hard bosses.
Then some class/race content to go with that, race based transmogs and perhaps jewlery and stuff that you can bling yourself out with.
If they had resources everyone could have cool content, the only reason it looks like there is a lot of content for "Hardcores" is because it's so easy to make. (Take the content you made for the wider playerbase, tune health and damage numbers 25% higher, add an extra soak to every mechanic and bam, you are done.