Oh Yeah, I see that. And those opinions would be just as valid to me as mine are.
My opinion however, is it isn't a
balance issue, but simply a lack of Spellcaster Class design that takes cinematic and episodic focus into consideration. 5E Spellcasters currently don't have the resource mechanic(s) that supports this type of play the way I would like it to. But their balance or lack of balance in comparison to other classes has very little to do with it IMO.
But perhaps we're talking about different things. When I hear
class balance, I think
"balancing classes to eachother". And I don't believe that classes being balanced to eachother or not, affect the 15-minute adventuring day issue.
The common ground in both cases is that the 15 minute adventuring day is a problem.
One, because it really doesn't work for a believable story in many cases. For balance, it leads to imbalance in favor of casters.
I definitely think it's important to split these - there are some forms of 15 minute adventuring days that are problematic to story concerns, and some that are only problematic for balance concerns.
For example, at one extreme end - if everyone plays a class with nova-capable resources that benefit from a 15 minute day, the 15 minute day is not a balance problem.
At the other end, you may have a session where the players are busy most of the day talking with NPCs, traveling or researching stuff, and eventually it leads to a violent conflict. Here, the balance problem arises, because that single combat is perfect for "novaing", but the problem for your story probably doesn't - it's not a literally a 15 minute adventuring day for you, since adventuring ismore than just combat and the party didn't hole up and do nothing.
By the way, this is also one way of dealing with the 15 minute adventuring day that is less sick and more carot - have the players spend the time not in the dungeon research stuff, talk with NPCs and all that. If there is a settlement near the dungeon that allows this, you may have less story problems with the 15 minute adventuring day. But you still have all the balance problems.
So to deal with the 15 minute adventuring day, we have to identify which is our concern and what measures will address which concern. Personally, I am primarily intererested in solving the balance problem - I feel confident enough that I can, if the story demands it, I can usually encourage (with sticks or carots) to motivate a full adventuring day, but I also know that I will have scenarios where there is only a single combat in a day, and that will be an issue.