There is still some content that needs to be added to the game, such as the vanity contest winners' costumes and apparently they will be adding more achievements. There's still going to be some updates after these apparently, but whether they are just final bug fixes/optimisations or if they plan on injecting the game with one last hurrah of content at the very end is completely unknown.
But yeah, Terraria is an old game to still be getting updates; the amount of stuff added post launch is almost unprecedented especially for a small indie company (though not so small now) and with a program that has been worked on for so long there is bound to be a lot of things holding them back (we've already heard this from Cenxx about how dying weapons is not possible due to how the engine handles particle effects). Plus, so many items that exist in the game that are severely underpowered/obsolete/the whole existence of summoner equipment being retroactively added to the rest of the core progression. I personally am pretty happy with this being the end of Terraria's updates, and I'd like to see what they can do on a fresh now project, and if they do decide on a Terraria "sequel" then it will be great for them to be able to take all of their newfound experience to make a more thought-out game as opposed to the hodgepodge conglomerate of a game we have today.