It depends on demand. If community stops using it favoring Ardor instead, it can fade away, and Ardor will carry on from there. Jelurida have expressed their intention to keep engineering support of Nxt, but it's up to the users what's going to happen. Honestly, Ardor supersedes Nxt beautifully and there should be no need for Nxt once Ardor is an established network, but again, users have the final say.

In my opinion we haven't seen the ultimate bubble in crypto, far from it. Cryptocurrency industry has yet to earn its first Trillion USD marketcap, and I believe it will be more than one Trillion. Ardor alone or with Nxt together, being unique, secure and mature cryptocurrency products, will get a nice share of this pie in the years to come.