My least favorite advice is when someone tells me to play another game when I want to do something in D&D. As a game designer who has worked on a host of systems via freelancing, from narrative to war-focused to D&D and D&D derivative, it really is not hard to run a fun social game in D&D, or an investigation, or a mystery, or cosmic horror, or whatever. It just really isn't that hard. And if I want to add mechanics from another game to D&D, it also really isn't that hard. PbtA is literally just roll over and depending on your level of success, you get different amounts of complications and gains. It's the most brain dead thing, you could make every skill check fit this, and why it may take like an hour for the GM to figure out some ideas, it just isn't that hard. Hell, most PbtA moves can be taken straight up and put into D&D and the only thing that's changed is the exact numbers of the threshold.
People who think that you shouldn't change the rules of a game or that you need to play a bespoke game for what's literally a small part of the overall experience are really telling on themselves as being close minded. Yes, if I want to play a game about supernatural teenagers in high school and their drama, Monsterhearts is better for that; but if I want to do a game where I sometimes go into dungeons, sometimes have to debate with royalty, sometimes have to hash out negotiations with orc troops, sometimes need to investigate a gorgon serial killer, Monsterhearts is freakin' useless. But with very, very, very little change, I can do that all in D&D.
Keep in mind that most of you grognards are surely aware that a lot of old modules introduce module-specific rulesets all the time. Saltmarsh's one quest that deals with talking to a bunch of reptile people and trying to build favor via a mechanized system is from the days of TSR and was reprinted almost word for word in Ghosts of Saltmarsh. If I want to play that one specific adventure about politckinh with reptile people in a marsh, should I jump to a PbtA game and then hop right on back to D&D when I'm done? This example illustrates the absurdity of this point.
I have also both played and run in several kinds of games that do "non-D&D things" while using the D&D ruleset. I've played in political intrigue games, I've played in games where we were roman soldiers with no magic and heading out into the wilds, I've played in games that were focused on figuring out who a serial killer is, etc. D&D's rules didn't get in the way of these games. For most of them, we didn't need new rules either.
It's just so wild to me that every time someone's like "Hey, let's talk about adopting this rule to D&D" or "Hey, let's talk about running something that isn't hack n slash," a vocal minority comes out to tell you how silly it is that you would ever dare to run something so """""avant-garde"""" in D&D.