There's a performance problem. The open Web only works because we have enough compute power to tie up multiprocessor computers with a few gigaflops per CPU and gigabytes of RAM to display 2D text with a few pictures on the screen. A good 3D game world pushes the limits of what current hardware can do. The level of inefficiency associated with Javascript/HTML/CSS won't work for an interesting 3D world. It was tried. See X3D.
Almost everyone has read or heard of Ready Player One (2011), which contains extensive descriptions of its own corporate dystopic metaverse, albeit one that I find insufferably cliche and unoriginal.
Metaverse descriptions are descended from the first cyberspace descriptions in Neuromancer (1984) which is a beautiful book worth a read.
Anything but an open and radically-decentralized interoperable set of standards and you're Running On Someone's Specific Hardware. That means that they and the country (or other municipal structures) might have opinions on what you can run on their hardware, and thus the Four Horsemen of the Infocalypse will be invoked at the start.
As you slide toward less interoperability and more corporate/government control, you're going toward the Johnny Mnemonic territory (maybe without the cyberdolphin). You'll have to hack your way around the place, and that means that the ordinary folks are left out. They might not have the smarts, the hardware, the wetware, and so on.
Further levels of control and you're into Second Life -- one platform, one company, but perhaps some local rules/anarchy. And if you're still thinking of the children, the Metaverse eventually looks like Club Penguin. You can finally rest assured that the concept is functionally dead when we are all sending each other only various emojis of unrealistic skin tones.
So, the question then becomes, does anything in the Facebook/Zuckerberg history suggest where on that spectrum we will land?
As a side note, I believe that pretty much any version of this Metaverse means targeted ads. On the controlled side, the people running the hardware will want money, and that means ads. On the uncontrolled ads, randos will want money, and that means that they'll do whatever they can to sniff your activity and monitor you, and then send you ads.