Have they given up on network effect of Facebook's platforms? If they can't solve network effect problem then I wonder how much hope there is for apps like Matrix or Signal.
P.s. We do have YouTube from Google but that's an entirely different story.
I don't think Google+ ever got to the scale where that was an issue.
I don't think that is the case. At least many users, possibly even most users, would not understand the difference between Apple hosting a social network themselves and Apple promoting a thin client that views decentralized content. If you have an Apple Decentralized Social Networks app, and a parent sees their child viewing objectionable content through that app, a bulk of their ire is going to be directed at Apple; they will likely not even know the name of the entity actually hosting the content.
The "we're not actually hosting it so we're not responsible" argument, I don't think it's ever really worked. I don't think it would work in this case. See: torrent websites.
I think the benefit to Apple also goes down. They give up control, and for what gain?
Heck, integrate it into messenger and treat all content like emoji/stickers you must acquire from a controlled source to circulate.
Legally maybe, but it's nevertheless presented under the Apple brand... which means as soon as the first pedos, QAnon cultists, antivaxxers and other undesirable elements take a look, people will reflect that negatively on Apple.
Apple's brand is basically to be "clean and safe" for users. Engaging in the shitshow that is any modern social network would be one of the fastest way to tank their stock price.
Apple was focused on hardware. Their services play is only recent and my best guess is if they ever considered social, they rightfully saw how toxic it is and decided it wasn't worth the effort/risk of tarnishing their brand. Not that I think they'd have a compelling product. Personally, Apple's web products are average at best. See their stock apps.