Many people are discouraging bringing old content to relevancy, but some MMOs like Maplestory used to be that old content continued to be relevant. As well as Toontown, which was a serious Disney MMO that was locked by town-based progression if you count that. Though they had different leveling systems and story progression in their respective games compared to Destiny.
I'm not sure what could be done, but IMO Destiny shouldn't have to give At-Level gear for old content, but saying that old content should be made irrelevant is not a good way of going about it. If Bungie wants to make the game more "MMO-like" then they shouldn't be content shifting, they should be content building. I feel like old content would have been more relevant if power levels weren't raised, thus giving players a reason to do old raids and old content. That aligns with most other MMOs as far as I know.
Though, I feel like Destiny has other problems. If this game wants to be more MMO-Like, then it should at least feel like a proper MMO and not just a glorified collectathon with RNG. Right now there's hardly any player-to-player contact or communication except for third party applications. There shouldn't be a reason to make it a pseudo-necessity to go on LFGs or Reddit to find people to play with. You should be able to meet people and form groups in the game. The Tower would be a great place to do that, maybe by a bulletin board or in-game chat if there were ever enough people in an instance to make it interesting to use that. The Tower shouldn't just be a place to go when you need to collect bounties. It should be a proper player hub. If not there, then a sub-section of the Tower should open up for this system. At the same time, in this zone, not require exiting to orbit to create new fireteams.
There's not enough players in an instance to make this game even be considered "Massively Multiplayer." Aside from the character building, story, and loot-shooter mechanics, I feel like this game is missing what makes an MMO great and fun to come back to:
The community in the game itself.
Right now, the game is forcing story content and collectathon-like systems to draw people to play the game and often times they make the collection filled with RNG and tedious. Man, I've really gotta finally grind out crucible for my Randy's Throwing Knife.
I love Destiny, but this game at its current state is just artificially creating play time for the sake of keeping people playing. Many MMOs do that too, but they have more substance behind them--more community oriented things to do and frankly, more story or unrelated short side story. Destiny doesn't have enough content to be doing that, and it's all they have been doing since the start. Why are strikes and nightfalls a repeat of story missions? Don't many other MMOs have side quests have a different setting or even a different path to follow? It doesn't have to be a completely new map. Why are PVP and PVE the only things to do together? Why not some more matchmaking casual mini-game mode?
Hell even with all this old content, why not create a competitive raid boss arena where you compete against other teams simultaneously going through all the bosses or mini-bosses. Or if that's too much time, how about a group of them? You know, something other than a PVP based competitive mode. You could even require that individual players finish those raids and dungeons before you can even access this gamemode. That would be positively encouraging.