I think bad endings (regardless of if they're explicitly called "bad" or are called the "normal" end but are severely anticlimactic or otherwise unsatisfying) are fine as long as either:
A) Going back and fixing things to get a decent ending is easy/quick enough (be it because you can straight-up rewind or because you can new game +/fast-forward back to the decision point)
B) You have to either deliberately fuck it up or be completely fucking clueless to get the bad ending
Persona 4 does this decently well in that it pretty much tells you to make a second save in case you fuck it up before you get to any ending-determining decisions. Atelier games with time limits have bad ends if you fail to meet any victory condition, but you have to
severely fail at time management to not get the baseline win condition. Most visual novels are fine here because they let you fast-forward through parts you've already read, so even if they are a "you have to do this random decision right in chapter 1 or you're locked out of the good end" deal it's pretty easy to re-play and get another ending. Ar Tonelico Qoga gets bonus points for its NG+ feature that lets you choose if you want to start at the beginning or right before its two major ending-determining branch points.
But yeah, plenty of games do fuck it up- I liked Omega Quintet up until the end where it went "HAHA FUCK YOU START OVER AND GRIND LIKE HELL IF YOU WANT TO SEE AN ENDING THAT ISN'T GARBAGE."
"Make sure you create a backup save before the last dungeon, then again before the final boss" isn't something you're taught in the tutorials, nor does it make any sense in modern gaming. For someone who might not know that sort of JRPG strangeness already it could seem odd. I mean, it's a holdover from games two decades ago. I wouldn't have it any other way. Though, playing Valkyria recently, I forgot how awesome it was that they left all the battles open during a NG+ so you could repeat them (and any cutscene) if you wanted to. More games need that.
Pretty sure P4 just about outright tells you to make a separate save before shit starts getting real. It's been a while, though, so maybe that bit was more vague about it than I remember.
On something vaguely recent, Neptuina V-II committed the sin the OP is talking about, though. I had to research after a full play to the bad ending how to actually get the true because I'd done every optional thing imaginable and gone through what looked like a bunch of standard flags. Turns out it's linked to one specific sidequest about a third into the game that takes a non-automatic trigger to even access. That is just not fair. There's no need to make it that complicated. Especially after the characters taunted me in the bad end for getting the bad end.
Eh, I'd say V-II doesn't count here because the "normal" end, while tragic, is satisfying enough. It's something that would make sense as a canon ending, which is how I'd differentiate a "bad" ending from a good/normal one. If the actual bad end (which you avoid by simply making sure you see four events right before a late-game scene) was that tier of Guide Dang It then it would be horseshit, yeah.
Also V-II has a pretty good/quick NG+, so replaying to get the true end isn't all that painful.
