If both parties have a vested interest in the relationship, each will adjust and change to keep the relationship of mutual benefit.

This is a bit like asking whether parents can have a valuable relationship with their children after they've grown. Well, the truth is that some can and some can't.

Or whether a marriage reaches an expiration date, or can continue to evolve in ways that enhance the partners. Some do and some don't.

I think it largely depends upon the authenticity of the relationship, the perceived benefits to the people involved, and the degree to which each person is willing to accept the risks of growth.

The only difference within a therapy relationship is if a point is reached when there is no longer any therapeutic benefit to be gained. At that point, the therapy should end. Whether the relationship also ends is a separate issue.