While GW2 development is not going to be as far as I would have hoped for, it is still one of the most successful western MMO games. I do not see a reason at all why it would come to an end anytime soon. The franchise is too valuable to just shut it down. There are many way smaller MMOs that are still online and/or in active development. Things like Everquest, Everquest II, Runescape, Lord of the Rings Online and even Meridian 59 (1996).
NCSoft is an uncertain factor in these times where most of their revenue comes from Korean and mobile games. But if NCSoft would ever decide that ArenaNet/GW is no longer fits in their portfolio, then the most logic outcome would be that they sell the studio or split it off. Just like LotRO changed hands a few times. (Turbine+Codemasters/Turbine+WB games and now Standing Stone games)
And believe me that LotRO is tiny in comparison to GW2.
And yes, a lot could be improved. I would love to see game/rendering engine improvements. WvW updates with the alliance system. Far better communication and involvement between ArenaNet and the community which would greatly reduce the amount of toxicity here in the forums.
But I'm happy with the expansion announcement, the continued living world releases and I am deeply thankful to ArenaNet for making the franchise I call home since 2006. And the fact that even GW1 still has its servers online and gets occasional updates reassures me for the future of the franchise.
I don't think a fourth expansion or a GW3 is not a consideration just yet.