We've asked Bungie to rotate vendor stock weekly since vanilla (they have reset timers for -blam!-s sake). Why Bungie won't is beyond me (except it's not, they want us to suffer thru RNG drops that never drop)
Currently they also have vendors capped at 390 drops because who the -blam!- knows why. So not only are bounties completely obsolete at end game, there's literally no reason to spend glimmer or marks other than to stockpile mats to level faster next LL increase (that they don't want us leveling fast so they do shit like cap faction LL drops).
It all makes glorious sense, if you're slightly insane.
The vendors light cap does serve a purpose in the fact that it forces us as players to experience other aspects of the game if we want to increase our light. After the april update I guess that is only true to some degree, but I can still understand the reasoning behind it.
As to the RNG suffering, I would say that restocking vendors would still make us suffer under RNG, it would just give us another avenue to suffer under. My argument is that it is fine, as long as it creates a more interesting gameplay loop. Which is what I think restocking vendors would do.
So much of the games weekly content is designed around the idea that Legendary Marks have value, at the moment they do not. Restocking vendors would therefore not only give marks value, but give value to the weekly activities that give legendary marks as well. It is a win-win situation for the game, at least from a design perspective.