Let's look at it a moment from 20,000 feet, at the larger issue for which the 15 minute work day is merely one famous example. I'm going the long way around for things that I think most people already know to show the linkage:
Namely, it is anywhere the game model breaks down, such that the effort required by the players is totally out of sync with the effort required by the characters. For example, Joe the Fighter goes on an adventure, kills some goblins, gains a level, and decides to put some skill points, feats, or the like into being a jeweler. This is almost trivial for the player, but is supposed to represent, in game, hundreds of hours of practice at working with gems and jewelry.
Now, you actually get the same effect no matter what Joe uses that XP for, but it's a lot more palatable that he gets a feat to hit a little harder with his longsword, as the training and practice are unspoken but assumed. That's why a lot of groups will tell Joe he can't buy that new or increased jeweler ability until Joe has spent some time in game on it. But even here, we aren't going to play it out in mindless tedium. Thus, in-game character time is a resource here, not, say, Joe's dedication and will to bend over small pieces of jewelry for hours at a time, day after day. Or more clearly, Joe spends "age" to gain "PS: Jeweler." Thus, if Joe is an elf that lives for a 1,000 years, and the campaign makes no particular effort to make time matter, then Joe spending "age" doesn't really cost anything.
And because these things are all "break in the model" types, they share with 15 MAD that they can't all be fully addressed by mechanics, but can be muted by mechanics and then the rest covered by advice. The game model will always have holes in it somewhere, as a game about killing monsters and taking their treasure will never be a great model for, say, medieval fantasy professions. In the case of 15 MAD, the game assumes a certain amount of desire and push to "go adventure" that you don't want to completely circumscribe with a bunch of mechanics driving the PCs to do that very thing. Encouragement and prods are good, fences and no real decisions, not so much.
So in general, I'd say that when the model starts to break down in ways like the 15 MAD, we want to examine the efforts and resources put forth by the PC in the game model, which are done in lieu of the more realistic efforts and resources that the characters might expend if their story was real. Thus the varied insights by many that part of the problem with Vancian magic is not that it is daily, but that it is daily that is easy to use, easy to get back. And that's also why the "solutions" are so varied yet not necessarily well received when shared across campaigns. The only solutions that work are the ones that impose real costs on the PCs in the model, whether that be by house rules, contriving events in the game world, etc. These necessarily change from campaign to campaign, and player to player, just as age is a real factor for Joe the human fighter in a tight, gritty campaign, not for Joe the elf fighter in a loose, epic campaign.
