I know that this is anecdotal and therefore doesn’t actually prove anything, but not a single one of my friends uses a full headset when gaming. The only person I personally know that does is my girlfriend and that’s only if she and I are both playing a story-heavy game where we want to make sure we’re not hearing the other person’s game over our own (our TVs are right next to each other).
Between finding the headset uncomfortable over long sessions and just wanting to have the ability to still hear what’s going on around me (my girlfriend’s dog likes to quietly whine in the corner when he wants to go out and if you don’t let him out within five minutes he poops on the floor) I don’t think I’ll ever be able to switch to a full headset.
There’s definitely still a market for a single ear chat headset, even if it’s not as large as it was in the past.