Do not keep a finished arrangement alive through vague promises of someday.

Soft endings are usually a kindness aimed at yourself. We should definitely do this again keeps the door open, avoids the awkwardness of saying it is over, and leaves the other person half-waiting for something that is not coming. A clean finish is harder to say and much easier to live with.

If something has actually ended, say so warmly and without a false horizon. Thank them for what it was and let the silence afterwards be honest. If you genuinely do want to continue, put a date on it, because someday is where relationships go to fade rather than to rest.