Forums are dying because they don't adapt to the new way of consuming content and communicate with other people.
Whether IPS, XF, vB, wBB, ... it's just not fun to use them on mobile, it's way too cumbersome. Besides, a responsive layout can't compete with something designed specifically for these devices.
I mean just look at this, nothing more to say.
Next point, why should I sign up on a forum when I'm already on Discord or wherever? And all my friends are already there too?
Forums will always have their place in some genres like support or other special interrest niches and for oldschool users who prefer desktop computers, but that are minorities.