In my opinion, the modern prevailing trend seems to be very anti-meta plot, and as long as that's the case, I'm of the opinion that people aren't going to be particularly invested in the IC trappings of the world.
If the world isn't moving and evolving with stuff happening in it, and people just make a game and go 'This is the world, this is how it is, and this is how it always will be', why should players care about it?
If staff don't want a game to be treated like a sandbox, then they shouldn't basically create a glorified sandbox to begin with. Effort has to go into making the world feel like a living and breathing thing. In my opinion, most other solutions are just a bandaid that ignores the root cause of the issue.
When a world doesn't feel alive, it's going to either be treated like a sandbox or as unimportant in general. Even in the case of staff not wanting to GM things, and want things to be PRP focused, if they want a game to feel alive, then people have to be allowed to run actually important and world affecting PRPs. Staff have to support and and treat PRPs as important and perhaps even allow players to create their own world affecting metaplot if you want a world to feel important while not GMing yourself.
I'm sure that my perspective on this will be unpopular, but if literally no one is doing any sort of important plot that has any particular consequence beyond that plot , no real staff endorsement, and no real feeling that one can affect or sincerely interact with the theme/world? People are going to treat it like a sandbox.
And I firmly believe that there's no way around that unless you make your world so small in scope that it's very simple to perceive change within one's environment without plots.
For example, a school based RP, like, an RP based around a single school, a single small faction, a single superhero team who has a base and leaves that base to do stuff outside of it even though the grid itself is mostly the base? Much, much easier for the game to feel alive with minimal staff intervention.
In my honest opinion, the larger the scope of the world that you want players to care about, the more necessary plot about that world is going to be. The smaller the scope, the more people feel that their day to day RP is a part of the world, with the option of going out into an even larger world.
I know that this might sound like I'm pulling shit out of my ass, but consider this: In, say, an anime high school RP, how difficult do you really think it would be for people to do PRPs that they feel invested in without staff putting their approval on it? Not very. It's also, in my opinion, much easier for players not in said RP to feel like it's something happening close to them, a school is a very tangible thing, people know that they know what's going on because their characters are in that school, even if they aren't in the plot.
A school, or really any particular interior location, is easier for people to buy into as important, as something capable of being tangibly affected, as feeling like a part of the same world as the other players. A city is a significantly harder sell. Because that is what you're doing, you're selling your location as something that should be invested and bought into, as something to care about.
The larger the location, the harder the sell, the smaller the location, the easier the sell.
I hope this made sense.