I disagree.
In Wrath, heroics were not exactly significant. They were your source of pre-raid gear, and you may have to farm to get the drops you wanted, but drop rates were much better then. After that, they were nearly meaningless. You rofl-stomped them early on. Yet, you still ran them. Badges and weekly objectives sent you back in there, although, less frequently than M+. They found ways to make them repeatable content.
They also usually added new dungeons for each raid for a new source of pre-raid gear for catch-up. These would be harder, but, you still overgeared them relatively quickly. Still ran them, at least enough for the weekly objectives.
The difference, IMO, is simply that M+ now is an alternate gearing path to raiding. Back in Wrath, you could play the world in heroic 5 man gear just fine, but it was a significant step down from raid gear. Now, with M+, you can be equivalently geared from just 5 man content. This is fueling player choices in how often to run this content. This is also affecting Blizzard’s design decisions on drop rates to keep the pace of power progression similar to raiding.
So, yes, now it is nearly impossible to feel “done” with a character for the week and play an alt. In Wrath, I could pump out my weekly heroics in a single day, farm some mats for feasts/pots, do an hour or two of rep dailies, and outside of raid night itself, that character was done for the week. Yes, that let me play alts, and boy did I. And yes, I preferred the game better then. But, that doesn’t mean M+ isn’t “content”.
The “problem” is that in Wrath, you could be “done” for the week on a main or alt due to lockouts. Now, with M+ being infinitely farmable, yet, having very low drop rates, there is always power to chase. This means any time spent on alts is time stolen from your main that MIGHT increase your main’s power.
I will say, daily obligations like rep are more time consuming than they used to be, which doesn’t help, either.
Whether you recognize it or not, I think lockouts are what made Wrath alt friendly, not a redefinition of what “content” is.
If anything, what I would like to see is dynamic drop rates in M+. M 2-5 should have better drop rates to gear you “pre-raid”, and have the drop rates lower above that, similar to how it is now. This would let me get alts into that M2-5 gear range, where, if necessary, they are ready for a guild alt-raid or to pug on a night the guild isn’t raiding, without having to farm the vault for 10 weeks to get there, and still keep them from getting truly powerful gear too fast.
I could complete a build feeling rewarded then make a new one. At least the corruption vendor was on rotation and could buy what you want and just make another build. In SL you can’t do that, you have to farm another Leggo pattern, grind torghast get soul ash, pay gold to buy the blueprint to craft it. Then farm the other gear.