If you rest after every encounter or two then you always have your best resources and HP, regardless of what edition or game you are playing.
The 15 minute workday applies to 4e as well. Your dudes could go in blow their dailies rest and do it again. There is nothing in the rules preventing them from doing so and it would provide a huge tactical advantage.
Not that huge, for a couple of reasons.
Most important is the change in the healing mechanics. PCs can be expected to be at or very near to full hit points at the start of each combat encounter (because they will have spent healing surges during their short rest), and the constraint on in-combat healing is not healing surges per se, but the capacity to access them (via spells, items, second wind etc). The fighter in my game has 14 healing surges. In a typical combat he will use 3 or 4 of them. From the healing point of view, then, he is no worse off going into a combat with 4 surges remaining than with 14. This is very different from earlier versions of D&D, where the player of the fighter may well be wary of entering combat with only 25% or 30% of hit points remaining - in those circumstances, one or two good hits from the enemies might kill you!
A second change is that daily powers are not as superior to encounter or at will powers, as are traditional D&D spells compared to mundane attacks by a wizard (low level clerics are good mundane combatants at low levels, but in classic D&D they also start to weaken a bit at mid-to-high levels). This is in part just a reduction in scaling between at-will, encounter and daily powers, and also because (at least for my group) the daily powers are often somewhat conditional in their utility, and so won't be pulled out for certain sorts of fights (for example, a daily which is strong because it allows three targets to be attacked typically won't be used in a combat with fewer than three dangerous foes).
It may be that my group is very idiosyncratic, but these changes have been enough to make the 15-minute day all but go away, whereas in our Rolemaster game - which, like pre-4e D&D, does not have a "reserves" approach to healing, and like pre-4e D&D has daily resources being much stronger than mundane/at-will ones - it was the norm. In Rolemaster, the players would rest once all spell points were spent, and that could easily be after an encounter or two. In 4e they rest once all healing surges are spent, and that genrally takes 4 or more encounters.
You say you have a problem with the 15 MWD, yet you encourage its use. My sympathies are limited by the fact that it is your choice.
If you are having fun playing that way, then you have no grounds for complaint. If you aren't having fun, then there are things that you could do to handle the problem, but choose not to.
That is not a problem with the rules, that is a problem with the GM and/or players. If you do not enjoy the 15 MAD then don't reward it!
*EDIT* I will agree that some advice on handling the 15 MAD should be in any 5e DMG. I just don't think that there should be rules about it. Just guidelines.
I have not had the problem beyond that one time, and then the problem fixed itself because I did nothing to make it a useful tactic. The bad guys weren't idiots, and took advantage of the fact that the 'heroes' decided to hide out for a little while.
As I mentioned earlier, this seems to be somewhat scenario-specific. In particular, it seems to posit the PCs tackling an active opposition, rather than something more passive. It also seems to require a willingness to have the PCs fail off-stage.
For those who (i) want the passive "opponent" option (eg ruins exploration), and/or (ii) want the "failures only happen onstage" option, then the 15-minute day
is a rules problem, given that - with changes to the rules - those sorts of scenarios and approaches can be made eminently playable.
Unless your game world is so frenetically fast that four or five months spread over twenty levels makes a difference, it just doesn't matter.
The ingame time for my group has always tended to be ridiculously short. For example, in the current 15 level campaign about two months of game time has passed - the party has only just finished rescuing some prisoners that they started trying to help at 2nd level, and they still haven't raided the mountain stronghold of the cultists and armies that they have been fighting since 1st level.
The actual maths of this works out pretty sensibly for a 4e game - 2 months equals about 60 days, or about 4 days per level, or an average of 2 to 3 encounters per day - many days would have more than that, and others fewer due to travelling, scouting, resting, etc. The frenetic pace has nothing to do with suffering or avoiding 15 minute days, nor with players or PCs being lazy or energetic. It's a consequence of the mechanics.
My group actually handles this tendency to freneticness through a type of "cognitive dissonance" between real time and game time: because those 15 levels of play have taken about 3 years, it doesn't
feel as frenetic as it in fact is in the fiction.
But given that the game is about as frenetic as it could be (and my Rolemaster games were no different) it's not as if I could, or would want to, add in furthert ime pressures to change the incentives for resting.