I mean, plenty of adults spend a ton of money going to see the PG-13 rated MCU films, so I don't think they're having any problems getting adults as customers. Even beyond that Disney "grew up" a long time ago...they own tons of media outlets, several of which have produced R-rated fare in the past. The primary Disney brand may still try to present a family-friendly front but as an overall corporation they're not squeamish.
So far Disney+ has been pretty strictly PG-13 rated (at least in the United States). It's not all super-kiddified but the violence is generally bloodless superheroic or sci-fi action (which you can have a pretty much unlimited amount of and still be considered PG-13 as long as it isn't bloody/graphic). The Netflix shows pushed those boundaries quite a bit and definitely veered into at least "light R-Rated" territory for violence and were also "sexier" than the MCU films as well. So putting them on D+ might be jarring for some parents who have generally felt that D+ was a "safe-ish" outlet for their kids. Not saying it's impossible, but I'm still putting my money on Hulu (in the States). That seems to be where a lot of apocryphal and/or adult-oriented Marvel stuff is going.