That's a very valid point. Playing an open streaming world over the web would result in Second Life effect. Where areas are white shaded blocks until textures load. There is very clear white out until then. From a solid game play perspective this is not acceptable. So playing open streaming worlds online is no good unless of course world building is that experience. So I still wouldn't consider closing the door on asset management.
On the flip side is that the open streaming worlds should only be designed local games. Ie nodewebkit or packed games for mobile. That way the assets are already all there to access and don'y need to be uploaded. Then there is much less load time.
As for unloading and loading of assets. Well cell like structure with let's say screen size + 50% dimension as a sample. The system would keep the screen and 1 cell size around the screen as loaded. When the player transitions a cell the farthest set then starts to unload.
As for teleporation white screen. If Teleportation is needed then use small subset of thematic graphics to each area that works as a world global set of assets. However since they only constitute 10% of each area.
So what I guess I'm saying is that letting players developers have access is important. If it's online world builder games were missing assets are ok and expected and if that's not acceptable such as GTA then the developer and designers can tinker ways around it by using controlled methods. however that capability needs to exist first.
However even GTA and Midnight Club suffer pop in effects. I'll use Midnight Club more. Midnight Club is an open world racing game. I'm pretty sure uses the same base engine as GTA3 back then. Players are given race tracks that consist of markers. however the players are free to get the markers any way they want. Early in the game popup isn't a problem. But I have to admit later in the game on the faster cars and bikes popups do occur. But seems to be handled by distance grid buffering. So even top end games still get hit by missing assets.
I'm not asking for an open world engine. Just a way to flag objects that use images that if there object count is 0, then unload the entire image set. By default this shouldn't happen. I think web focus should have priority. As for asset loading.
If game designers set goals such as no more than 2 thematic areas at one time. Then world design can take that into account and avoid issues. Developers are crazy nuts that can figure out possible solutions. Providing there isn't anything blocking them. However memory overload is a blocker.
And as you say since WebGL can't do asynchronous loading, and at this time looks like WebGL can't do webworker ascyn loading. Then there is a blocker in the WebGl. So not much can be done at the moment. Pity. But maybe in 6 months.