It happens rarely, but the last time I had a TPK, it was pretty grim. All the momentum was sucked out of the proceedings, and everyone seemed demoralized. The party had spent months working towards their goals, actually setting up a base of operations, and had several irons in the fire, and now...they were all dead.
I offered a retcon, or "they had been captured not killed" or even "revived but with consequences" and the answer I got was "we'll get back to you".
And then everyone just got busy at the same time. The game had died and I was left wondering what had gone wrong. Had I overtuned the challenges? Should I have softballed things?
I generally don't, I roll in the open, and I'm very transparent about what enemies can do- I've had DM's who lost my trust in the past, and once gone, you can never get it back, so I never fudge or withhold information more than I feel I have to. That way everyone can make informed, tactical decisions.
Heck, one time a player was about to make a mistake and I offered them an Arcana check to realize that their attempt to Command a Gnoll was going to fail because Gnolls only speak Gnoll, and the spell is language-dependent.
In the end, it was just bad luck, but it didn't matter. The players felt cheated out of the fun they were having, and I didn't know how to recover from that. I still don't, really. I've heard suggestions, but nothing "feels right" to me.
So the idea that the game is designed to let players succeed doesn't bother me. The way I see it, that's just a safety net in case I build an encounter that ends up rougher than I'd planned.
I mean, I can always make things harder for the players. That's easy. Find ways to not let them rest when they want to, use terrain and traps creatively, make mixed groups of enemies whose abilities synergize so that combat is more like a puzzle of which enemy to take out first, extra objectives required to "win" a given scenario- that I know how to do.
But how to get a game on track when a lucky crit obliterates a player character who is vital to success? Yeah, that's hard.