I wouldn't consider the majority of the Tekken characters pointless, maybe ancillary to one of the storylines, but not pointless, as the main storyline is it's a fighting tournament.
It's pretty difficult to put on a fighting tournament with only 3 people, a grandad, his son & grandson, even if it is organised by the grandad's company.
If it wasn't for the other characters, it wouldn't be the "King of the Iron Fist Tournament", but more like an episode of Trisha or Jeremy Kyle, The Mishima Family Feud.
The game's a fighting game based around a tournament, so you have to have more than 3 characters, 3 generations of the same guy. Every character has a legitimate reason to be there since they're all invited to/enter the tournament.
If it were a JRPG about the Devil gene & the Mishimas, then maybe the rest of the roster would be pointless NPC's, but it's not, it's a fighting game.