I’m primarily a DM as well and I don’t mind losing DM fiat. I want my players to point out when I’m not following a rule because I don’t have rule mastery as I used to. I hope I have gained the trust of my players that if I’m not following a rule, I have a reason for it. I agree that not having any DM fiat means they are a player! 
I think the Tomb of Horrors is the best example of why DM Fiat is bad. Gary wrote that to prove that no one was perfect and to destroy characters. That’s all it was for. For wargamers, “beating” a scenario was probably fun. For me, that’s a waste of time. Again, different things for different groups.
Does any 5E monster scare me? Yes. Ghosts or any creature that can do a possession. 5E seemed to return CHA to a dump stat and to get rid of possession is a CHA save. It’s not even that high of a save, 13, but if failed, they can only get rid of it by someone turning it, the ghost wills it, or reducing the body to 0 hit points. Otherwise, it’s permanent! A haunted place with more than one ghost attacking the party to try and possess them will eventually do that. The bad die roll will happen. Then what? In general, though, I agree with your point. 5E monsters don’t scare me.
I wasn’t confident enough when I was running 1E/2E to modify things or stand up to my players. I could now but I don’t prefer those as a base system. I would heavily modify them so as to not recognize them.
I currently change monsters as needed. Not during a fight. If a fight fails, if possible they learn from it.
I personally don’t have a problem with ideas for an adventure. I have problems with ideas that will work and are fun. For example, I came up with what I thought would be a fun campaign where the players can only start with Core PF1 classes and if they wanted to become a Magus, for example, I would push them to multi class and then have them be the ones that found the way to be a Magus and switch all levels over to it. Even before then, I said they would slowly get some of the Magus abilities, as it was central to their concept. Oooo, they didn’t like being restricted in their choices. My players tend not to like political campaigns, so I have to keep that to the background. I still find myself doing a bit too much with politics at times. And so on.
I never liked “save or die” powers. Same for level drain. One of the things I had to learn was that this is a game and people, especially my group as middle aged adults, are looking for something fun. Those things are not fun. One of the things that 4E did right was the gaze of a medusa wasn’t save or die. It took two failed saves in a row, with penalties for each step in the process. It still took the character out of the combat which was the point.
I appreciate that 5E orcs and other “low level” monsters are more viable at all levels, just need a higher ratio of the monsters to players. It never feels like heroic fantasy to me with 5E, where a mid level group can’t hold the bridge/pass/chokepoint against the hundreds of orcs. They might be able to for a while but not more than an hour. In contrast, a mid level 3E/PF group could hold that pass for quite a while, depending on some spells and healing available, and a high level 3E/PF group would just destroy all the orcs. Again, that’s me.
Thanks for the replies!