When gamers try Second Life, they complain that it's s-l-o-w. That's a fixable problem for people with gamer PCs. They have enough CPU and GPU to do this right. The classic clients are single-thread and OpenGL, so they can't use modern gamer hardware effectively.
If you get rid of the sluggishness, user acceptance improves considerably. Or so says the literature on game design. You get flow. Performance and graphics quality alone are not enough for a win, but not having them means a lose.
I don't have all the answers. But I've pushed things forward a bit.
Lots of people are now making noise about metaverses and AI. Unfortunately, most of the people making noise are the same clowns that pushed NFTs. But there's some good work going on. This has the potential to make metaverses less empty, and human users will have NPCs with which to interact. It can definitely solve the emptyness problem.
NPCs are getting smarter. One of my NPCs in Second Life has a T-shirt that says on the back "Trainee - Someday we'll be in charge." That's less of a joke and more of a threat now.