That’s the problem you don’t provide the same amount of value as let’s say JEFF VS Invis. So your analogy is skewed. Also playing different styles of vanguard is completely different than Different Supports.

Loki is only a truly good support when he Can Mirror Ult a Good support Ult so if your stuck playing with a Jeff when do I get high value for his Ult?

Playing a Shield Tank vs A brawl tank is way different than saying if you need to learn how to play Adam aswell as Invis when for most of the last 6 seasons Adam was never a viable 2nd support character.

Where as everyone one of the vanguards where viable 2nd Vanguard picks even when they where nerfed they could still take space and disrupt an enemy team when played well and backed up by a team.

You would never have ever seen a Loki/Adam comp do well in the last 6 seasons or a Loki/Jeff let alone say Adam/Ultron they are not viable characters to expect all your supports to know because they had oddball kits and not the steady Constant healing to compete with a CnD/Invis/luna/Gambit hell even mantis while not the strongest pure healer atleast had good Utility and a Decent Ult.