The user base is healthy. https://destinytracker.com will increase by a lot once Forsaken drops.
If you haven't played D2 I highly recommend it. One of the best games this gen.
It should be worth noting that it's not actually free. The base game is literally unplayable without the DLCs, and you can't even play the Forsaken DLC unless you buy the other DLCs.
Same old money-hungry Activi$ion.