In my experience, good fights aren't always about winning or losing. In fact I'd argue that fights that rely on reducing the other side to 0 hitpoints can be very boring. As a DM I prefer to make different stakes than that for my fights.

Fights were the party can easily win, but shouldn't fight at all.

Fights where the party will surely lose, but fighting may save the village/town/city/castle/nation (think 7 Samurai)

Fights were you should fight for a round or two to delay the bad guys, but retreat or be overwhelmed.

Fights where you should blow resources to win quickly because the clock is ticking, even if you know it will leave you blown before fighting the BBEG.

So far I haven't had a TPK, but it could very well happen. When It does it will be my job as the DM to make sure it is as fun and memorable as possible.