Well, call it anecdotal but instead of people actually looking for other people to play, trade chat at least on my server is full of boost raids.
Point may be because a lot more people can do it than previously but i the overall relevance of heroic raiding has dissipated, my Hunter never touched heroic once yet still has like 402 Ilvl and barely needs anything from the raid except trinket(s) and some specific azerite piece.
The incentive to raid anything below mythic is just not there anymore in terms of character progression.
Previously, if people didn't like Pug raids, they had to organize to at least a 10man raid in order to progress their character, now four other people and some decent keys suffice.
I wouldn't blame the M+ chest itself, it is however a factor.
People can aren't raiding mythic?
Personally i would exclude Mythic (raider) from this discussion entirely because, as said multiple times, their incentive in terms of progression / motivation isn't affected as much as compared to someone who raids below mythic.
That statement was never implied in any form.
The issue is just that unless a certain activity has a decent chance of providing an upgrade, people are less likely to be motivated to engage in an activity, one that requires actual organization on top of that.
If your character already is decked in 400 Ilvl pieces, the motivation for heroic raiding is just not there anymore for a lot of people, Titanforging does not fix that.
You could turn this around and let M+ only hand out really bad loot, then watch how the participation drops through the floor.
We had the same thing in Cata / MoP, 10man Heroic yielded same loot as 25man Heroic, yet people slowly but surely gravitated towards 10man because the organization part wasn't nearly as big as for 25man Heroic.
The reason why this effect wasn't nearly as big during Wotlk because 10man loot was made intentionally worse than its 25man counterpart, thus giving an incentive to push into 25man.
And this statement has shown not to be true for anyone who has actually played TBC.
Mechanar heroic was among the most popular 5man dungeons during TBC - was it however the best designed one?
No, it just yielded a lot of Badges for a rather small instance, thus many people made sure they ran it least once per day.
Similiar story with PvP in TBC, a lot of people just engaged in both Arena and Random Battlegrounds because the PvP Gear was pretty decent even for PvE.
On top of that, it was easier to acquire than any equivalent PvE piece.
Once PvP and PvE had a clear separation, participation rates dropped during Wotlk, because PvP gear was pretty bad for PvE.
The idea that people simply choose the type of content that is most fun to them is not true in a progression based RPG, because the progression itself is fun, not necessarily the activity.
The quintessence is that progressing your character is still fun, even if the activity you are doing may not be fun to you if viewed in isolation.
M+ is simply more efficient than heroic raiding, because a lot of the organization effort vanishes.