I have been struggling with this very issue since joining Medium. Before I became a serious Medium writer, I had already started six blogs and I wrote for HubPages and other content sites. I had six different blogs and kept starting more because I have so many writing interests and each blog was supposed to be in its own niche. I also am a contributor to a product review blog. It was started by an old friend from Squiddo and many of us contribute to it.
I have been neglecting most of those blogs on a regular basis since joining medium. Now I’ve even been neglecting Medium since I went back to bookselling in July. But as you say, I have spent a lot of time and money on those blogs in the past and I don’t just want to take them off line.
Also, I don’t believe in putting all my eggs in one basket, especially if I do not own the basket. As soon as I have time I will probably shut down a couple of my blogs I have lost interest in. I just need to figure out the best way to do it and retain the articles somewhere that are worth retaining. Many are now obsolete. I need to learn the art of redirecting. What I hate about blogging is all the technical things that need to be done. One reason I love Medium is that they take care of all that. I also need to update my work on HubPages. That site still gives me residual income that can be very useful but many of my ten-year old posts have become obsolete now.