But that's the problem, then you're stuck in just that area if you want to fight enemies at your level. That gets repetitious, and it would be nice to have a reason to go to the other area.

Only in "Explore" mode. High level players also have the option---if the data-mining is correct---to re-play missions and strikes at very high levels...and to do the raids. Which is possible to do, because you already get what amounts to private servers for these "respawn restricted" areas. But to do what you are doing would require a whole new set of servers for what are now the public areas as well. I don't see Bungie doing that. The kind of things you are suggesting are part of the reason why so many MMOs charge their players subscription fees. Because it provides a steady stream of revenue to allow expensive upgrades of the sort that would be necessary to implement your idea. Not charging them, and only limiting themselves to the $60 price of the game, limits what Bungie can offer in terms of play opportunities.