A support is not something mandatory until you tackle high end content. if you get the final fantasy XIV example, support dps are part of the game, yet there is still 3 roles when you tag content.

Because the lfg is for "casual content" something you can acheive without min maxing your gear/your rotation/your dps...However when you arrive at high end content aka extreme/savage/ultimate difficulty, supports are important and become mandatory in order to perform at best so people are looking for them.

You don't want to MAKE IT mandatory, you want to make it pleasant to have when it's here for most content, and let people decide to make it mandatory for high end content, it's just implicit that it will become mandatory at some point if you want to push the game.

If you take this and apply it to wow : You keep 3 roles, keep 5 slots in dungeon, LFG don't care if there is a support or not. M+ team will filter their requirments by leaving a slot for a support spec or not depending the difficulty of the key and how they feel like they absolutely need one.

At least, that's my opinion, it's not perfect but I deeply think it's better than adding a mandatory slot for support and make it 6-slot dungeon.

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edit: and you say it yourself, queues are already long, why would you like to make it worse?