People complain about trinity but it works fine.

The problem with support is that unless it can cause failure or success, then it's irrelevant.

Tanks, healers, and dps all have critical roles.

Supports have no role. If their buffs increase dps, then they're effectively a dps class. Their buffs/debuffs will be balanced so the group does the same damage regardless of whether the group uses support or dps. Which means support is irrelevant and might as well be a dps class.

If buffs are needed for tanks and healers to do their jobs, then the tanks and healers are underpowered. Making tanks and healers weak so they need support just makes things harder on them. Tank and heal are already unpopular. Don't add support and make them even weaker and less popular.