There should be a caveat here, I think. Bad guys have to survive contact with the PC's in order to react to their efforts. Now maybe other campaigns are different, but in the ones I play in, there aren't often survivors to tell the tale...
Plus, how quickly one can react is questionable. If I run an encounter with my PC's and they are stymied by the resistances or special abilities of an enemy, even if they can somehow run away, what can they do in a short term?
You'd need a long rest to change out spells, if your class even allows it. You can't just retrain feats. If you need different weapons, you need to go out and acquire them (ditto with any sundries, including scrolls and potions). This all takes effort, and isn't going to happen overnight.
I've heard a lot of people justify, for example: "well monsters know to finish off fallen foes because Clerics" and then I go through encounter after encounter with no NPC's healers in sight. Or having NPC casters just randomly have scrolls of Dispel Magic to foil some strategy of the PC's- where did these scrolls come from, and what did they have to do to acquire them, and why haven't they needed to use them on something else before now?
It's ok to make changes to the game in order for it to continue to run smoothly. But you can't just say using DM fiat isn't using DM fiat. I once played a Fire happy Sorcerer who did lots of bonus fire damage (PF1e game) through various means.
Suspiciously, a lot more enemies with fire resistance showed up, which came to a head with a group of mercenaries who all had protection from fire potions ready. The DM was like "well, people have heard of you, so they know to prepare..."
And I was like "First of all, who, among our enemies has been spreading this information? Our last adventure had us fighting undead for a week! What, did some ghost go whisper to the mercenaries "hey man, you better have fire resistance to fight these guys"? And also, the last time we were in a major city, we were barely able to find 5 potions of healing- how did these 12 guys each get a potion of fire resistance?".
If he felt he needed to use these tactics to deal with an extra potent Fireball decimating his encounters, well, he's the DM. I guess he's gotta do what he's gotta do. But don't try to snow the players with a voodoo shark (an explanation that just raises more questions than it answers)!