The DMPC is almost as notorious as the Mary Sue. I could see that going particularly badly wrong in 5e, with the DM called upon to make so many judgment calls and rulings.
This is sort of a false attack on the DMPC, since
all characters are typically mary-sues. There are many people who outright state they have no interest in playing anything other than themselves in a D&D (or other RPG) fantasy. I don't blame them. We all live typically mundane lives in a mundane world where we eat, sleep, work and then we die. So when we step into a D&D (or other RPG game) we want to do all the things we can't do IRL: sling spells, call upon the gods, clash swords with demons and typically people want to be pretend it's
them doing all this, even if their character is called Rastlin or Tanis or Tasselhoff: David Tasslehoff.
Sure, the DMPC creates problems when they're doing things the rules typically wouldn't allow the players to do, or things that don't exist in the rules at all. I've played with those people. I'm
still playing with some of those DMs and it is annoying. The idea that bad gaming is always worse than no gaming is completely untrue since it's not a black-and-white experience.
Playing a 5/10 game can still be more enjoyable than playing no D&D at all for 5 years. Turn that around and then only get to be DM for
another 5 years. Eventually, you're so desperate to play you'll take that 5/10 game with the bad mary-sue DMPC and the weird furry overtones.
A DMPC can be just fine as long as they
stick to the rules. Sure, they're probably going to plot-device it up a bit, but provided that beating the bad guy doesn't rely on the DMPC being the chosen one, who cares? If you're concerned about "rulings" then just don't do a lot of things that require new rulings. It's not
hard to do. If you do, use them as examples for other players do follow. "Hey I used *special skill* to jump across the cavern, it worked for me so it will work for you!". Apply your rulings fairly and your DMPC will die when other PCs die, and your PCs will get to do cool stuff the same way your DMPC does.
Sorry this is really just a pet peeve for me because I often DMPC and I make a concerted effort to behave. Because I want to play far more than I want to DM and I haven't played in long enough that I'm only a hair's breadth away from playing in that game with the weird furry overtones and the mismatched-eyes and striped-socks DMPC. But DMing seems to be the only way I get to ensure the quality of the games I'm in and NO, DMing is not a substitute for playing, it DOESNT satisfy the desire to play and it's fun and rewarding in a completely DIFFERENT way.
DMing is not playing. It never will be. One is not a replacement for the other.