If you've had them sitting for 3+ years and haven't considered using them to fly DL or JB or with another transfer partner, either because you don't fly a lot or these airlines don't work for you, perhaps your best bet is to cash them out for a gift card at 1 cpp to a place you frequent. It's obviously not the best value you can get, but it's still better than getting nothing at all and just having them sit there since you don't seem to be adding many points to your balance due to very low card usage.

What was the offer of 15k points instead of a statement credit that you weren't aware of? It isn't clear to me in your initial post how you could have earned those?