Hmm, so they do not even ask for the reason why the danger estimation of the dungeon was raised?
Yes, the first step to doom is ignorance.
Also, let me understand the part about dungeons correctly:
1. Maintaining the former temples and the like with maybe a few dozen workers - as in cleaning and such - is TOO EXPENSIVE because they are too large?
<--> But having several dozen of adventurers go into a dungeon every day, fight monsters and individually get paid more on average than any worker cleaning and maintaining the building would ever get, is financially worth it?
2. The monsters come to exist by mana and miasma gathering around certain items and stronger monsters give more valuable items.
That means when the people abandoned those buildings "because it was too expensive" they actually left treasures like valuable rings, gold, accessories and so on behind?
Otherwise there could not be any monster dropping such items, right?
3. If you abandon such a place and you know that it will turn into a dungeon due to that later on, then why do you not simply demolish it ahead of time and fill the site up with dirt or flood it?
Then you could avoid the whole issue about a dungeon being created.
If there is an issue about miasma and mana gathering there because it was a former religious site then simply pay the church or whatever to perform a purification ritual once a month and the issue should be resolved without a dungeon being able to form.
I seriously like these fantasy stories but sometimes when the authors try to give explanations about those fantasy worlds I am surprised that they do not even seem to think about what their "explanations" mean before using them.