Letting another author know I'm using one of their results
Citing another is in no way "leaching". It is the natural course of scholarship. It took a long time for my dissertation to be extended, but I was very happy when it occurred. I was, in a small way, an inspiration for another. That is what we do in academia. 

Send the email. Moreover, wait and hope for a similar email to come to you in the future. Too seldom do we get to actually thank the people on whose work we build. 

And if the person is still working in that field, asking for collaboration is completely proper. In my own example, I'd left the field and had nothing to contribute going forward, but it was still a nice feeling that someone, somewhere, still cared.