I've been in a similar situation and yes, got another, older rescue. In my case, I wanted the newcomer to be a playmate for my younger dog, who was running the old boy ragged with her youthful energy. But I didn't want the old fella's position to be usurped, so it had to be a dog that would accept a doddery deaf old buffer as head dog!
The only word of caution I would give is to make sure you take your own preferences fully into account. I stupidly looked more at how the dog would relate to the old dog and young bitch so that the dynamics of those relationships would work and did not do a thorough enough job on assessing the newcomer in terms of how he would relate to us and our life.
But if you make sure your new dog fits in with you as well as with Toby, I can't see a downside here. Except, of course, that you could well be in the same boat again in a few years when Toby's time is up. If continuing dog ownership is not a problem for you, then why aye, go ahead