I didn't understand anything in the post above and I'm only four beers deep 
The modern MegaTen battle system is great by itself but it really doesn't work well with the traditional JRPG dungeon design of balancing tactical thinking in battle with strategic thinking outside of battle. The mental fatigue and "battle high" you carry as a dungeon progresses means that your ability to tolerate the crap that's bogged down the genre since it went 3D (e.g. loading, animations) diminishes considerably and instead of being a pleasant experience it becomes an exercise in endurance. A solution to rectify the situation was to make the dungeons more of a sprint race than a marathon with the the Dark Hour conceit but this ends up feeling like a band-aid instead of a solution.
Seems like an incorrect hypothesis.
SMT games are at their heart dungeon crawlers.
SMT3 has no safe places, because you are in hell on Earth. So the entire world is a giant dungeon.
Digital Devil Saga games are more proper dungeon crawlers.
Most jrpgs have a structure of town -> dungeon -> town -> dungeon. They use the dungeons for battling and challenge and towns for story exposition where you can take a break from battles. DDS doesn't have this structure. You're practically always fighting in a dungeon in that game. There are towns but they're usually one screen towns and mostly meant for unlocking crap and locations related to progress, and you're back to dungeoning and battling. FF13 is like this as well. There are no towns in that game, it's mostly just battling, and people get fatigue. Further, SMT3 has that town -> dungeon structure but the twist is that towns have battles too so there's no place to wind down. What you're experiencing is something common in complaints for people who do not like dungeon crawl battle heavy RPGs and prefer a more traditional structure that has starts and stops where story isn't seen as a reward for finishing a dungeon but merely a part of the game structure as much as battling and dungeoning.
That's where we get into Persona 3. Persona 2 games are both more in line with the traditional rpg than the MegaTen dungeon crawl aspect I mentioned above. That isn't a band-aid. It's just P3's way of being a traditional rpg and a subversion of town -> dungeon -> town -> dungeon. This is also generally why Persona is more popular. Because it's more traditional and kinda always has been, and easier to digest.
Personally, I love the aforementioned MegaTen structure. Constant battles mixed with light story exposition and telling story through themes, symbolism, and environment, and making it where it counts (your choices as a player) fuck yeah. And DDS has some of the best dungeons in an rpg ever. Compared to FFXIII and its static always hold up dungeons we are talking the difference between cream (SMT) and actual crop (FF13).
The fatigue you're talking about is more from the fact you probably aren't a fan of the structure. That's okay though. Not everyone is. It's quite true that SMT3 and DDS are games that battle endurance, but what you miss is that's the entire fucking point. Being stuck in a dungeon, overwhelmed, low on items, low on mp, and feeling overwhelmed by the pressure of continuing on is the entire appeal of that type of game. Not many people get that. It's a totally different rpg, for a totally different type of rpg player. You can throw Breath of Fire: Dragon Quarter and Valkyrie Profile 2 in the same barrel. As it is, the hypothesis that SMT games need less battles is an incorrect one, because battles, endurance, and fatigue are the entire point of some of them. It's like saying Fallout games need less choice because you're overwhelmed by the amount of choices, when that's the entire point of Fallout.
Just a different type of rpg.
PS2 rpgs the GOAT
