Ryllau, I find myself in almost the same situation as you. I've owned Theil CS3.6's since 1996 and have been exceedingly happy with them. I've also made the decision to upgrade, and am questioning my sanity because of it. I do realize that there are better speakers out there and that technology has progressed significantly since the 3.6's came out. My issue is that I'm finding that to better the 3.6's, I'm most likely going to go with the Revel Salon's. They're the only speaker out there (under $20k) that I can find that convince me in all areas that they're a clear step-up from my 3.6's. Except for the Revel's, I haven't found any other speaker that I would give up the 3.6's for (at least if I have to lay down $10k+ to upgrade).
So, what speakers to keep for life? I'll be keeping my 3.6's to put in a secondary system, thank you. They're bigger than I would prefer for a secondary system, but when I consider how hard they are to better, and that they are absolutely flawless in every way but would only bring ~$2k if sold - then there's no way I'll get rid of them...until they give up the ghost. And that'll likely be a long time from now.
So, what speakers to keep for life? I'll be keeping my 3.6's to put in a secondary system, thank you. They're bigger than I would prefer for a secondary system, but when I consider how hard they are to better, and that they are absolutely flawless in every way but would only bring ~$2k if sold - then there's no way I'll get rid of them...until they give up the ghost. And that'll likely be a long time from now.