My guess? I'd think the film got pushed back again because the studio is banking on the extra time allowing for more butts in theater seats; more time for the pandemic to ebb, more people to get vaccinated. And it's going on Disney+ because we'll still be in a gods damn pandemic and even if things don't go totally sideways by the film's release date, a whole lot of people will still be wary of going to the movies. And D+ has been, from all reports I've seen, stupidly successful and the MCU shows have been stupidly successful.
Widow's not going to make as much box office as other MCU films have (I assume). It's still a f*cking pandemic, people are still staying home. And if we're still knee deep in all this by the time Eternals hits, that film won't do huge box office either. So of course they're gonna throw the film up on the biggest streaming service around right now, which has already proven it's worth and made a crap ton of money. Between traditional theaters doing, like, half normal attendance or whatever they end up pulling, and everyone and their mother having Disney+, the film will be perfectly profitable. It's box office will be great for the 2020-21 season but awfully low by pre-covid standards. I wouldn't call that bombing, but saying the film won't do huge box office numbers seems a reasonable prediction to me. That's only part of where the money will be coming from, and quite possibly the lesser part, though.