When I press team up and find a team either someone ends up leaving or they don't tell the truth about them doing the full dungeon. I wish they would make that stuff for people to solo because its important to have for the game.
The game is targeted toward families so naturally they make dungeons and other content that encourage teaming up with friends. Unfortunately many people do things that make it less enjoyable which can be annoying but it isn't the fault of the devs. I've had bad teams so so many times but more often than not I get a decent team, I hope you start finding decent people so you can enjoy the game! :)
A lot of the content within the game is 'soloable,' if that's a term.
The interesting pattern here now is that many of the excellent gear is now locked behind side dungeons which as you eluded to, usually requires a cohesive team.
Perhaps another indication can be added to longer dungeons in which players are forced to commit to doing the full dungeon or not.
If they opt out, the punishment should probably be greater than a mere '10 minute penalty.'