As someone who enjoyed the game even with the "level 20-29 grind limbo", I have concerns about the longevity of the game. My concern is that Destiny's popularity and high level of gamer engagement has only lasted this long because of the game drought we are in. I think this continued struggle for identity is going to cause long term problems and here's why...
Players looking for a good PVP experience will almost certainly never be satisfied. Why? Because PVP in general, across all titles, is incredibly repetitive (this isn't Destiny's fault). And even with special events like the Iron Banner, if we are honest, the PVP experience in Destiny leaves much to be desired for (lots of lag comp, delay, very little depth, typical game modes, etc.) And you really can't reverse or undo the terrible experience so many of us had with OP shotguns and the misleading nature of the first Iron Banner. So all your die hard PVP fans will eventually jump ship when better big name PVP titles start to drop. Especially games whose identity is more firmly planted in PVP. They tend to offer more and have deeper layers of creativity and incentivized reasons to play, level up, etc.
Players looking for a good PVE RPG shooter experience have already moved on because there is little reason to play once you get to level 28-29. Continue grinding for materials? Waste 3 hours on the Vault of glass only to get no vault armor? No thanks. Many of us have gone to Borderlands The Pre-Sequel and many more will move in droves when bigger next-gen RPG titles like Dragon Age and others start to launch.
The reason games like Halo, Call of Duty, and Borderlands have been so successful and have high DLC saturation rates is because they have a clear identity. So expectations are set and largely met, resulting in a loyal and recurring audience. Destiny has tried to straddle the chasm between the experience of a solid co-op open world RPG and the experience of a fun fast paced competitive shooter. In the end, both aspects of the game are diluted, repetitive, and boring. Again, as I said on the outset, if you aim to appeal to everyone, you appeal to no one.
My recommendation is this. Since the nature of PVP games are so quickly ran through and cast aside (see Titanfall and each Call of Duty title for examples), I would strongly suggest focusing all efforts on content. Content content content. Content is king. Competitive shooters are going to re-saturate the market soon, and there is going to be very little to make Destiny stand out, other than the litany of problems and complaints. Leave PVP as it is and continue to add bounties, but if you really want people to play your game for ten years, you need to leave the PVP on the shelf and refocus your identity. Not because PVP is a dying genre, but because it doesn't line up with the identity that Destiny promoted itself to have and it doesn't have the longevity you're hoping to get out of it. Go back and watch all your teaser trailers and announcements and you'll see why so much of your audience is unsatisfied and angry.
See Borderlands as an example of an RPG shooter that has ridiculous hours logged from their audience, and launches DLC for almost 2 years after the game drops. They wouldn't keep doing it that way if it wasn't working. Then look at Call of Duty. A new title every year with DLC crammed out faster and faster each time (and constant marketing to get you to purchase DLC before the game is even out). If Call of Duty has to launch a new full game every year to stay on top as the biggest PVP shooter, and they even had two different companies so this was even possible from a development standpoint, you're going to find yourself running up a mountain on a treadmill. You will never keep up.
Reestablish and fix the identity, and set yourself up to have a loyal and recurring audience. Anyone in business will tell you that loyal and recurring business is far better.