It wouldn't. People would just run them with friends or not bother with them at all. DPS queue times are long. Most people want to be DPS, it's that simple. Let's compare the party compositions for dungeons (for example levelling dungeons in both games) between WoW and FFXIV
WoW: 1 tank, 1 healer, 3 DPS
FFXIV: 1 tank, 1 healer, 2 DPS.

We have a much higher DPS playerbase, let's assume they're equivalent across both games (percentage wise, let's argue 10%T/10%H/80%D). Less DPS are needed for each instance, which means to fill all DPS into a group, you need more instances, which means you need more tanks and healers. Yes, DPS have long queue times in all games, but it is especially exacerbated in FFXIV by the party composition.