I jumped to Studio One as well.  Nothing new will happen in Sonar at this point, as I'm spending most of the time getting familiar with S1. This is as good a time as any to migrate for me.  I also don't like the idea of using a program that I know will no longer advance and keep up with technology.  That's not to say it isn't a great product now...which if course it is.  Potentially much better than Presonus.  But I believe in the near future, technology will leave Sonar behind while the other DAW move forward.  
If I didn't want to advance technically, I'd still use tape machines.

I feel . . . it's a highly emotional decision not to use Software that is absolutely up to date and will be for the next one or two years. I don't see any need to rush things.
That doesn't mean I don't look around and try other DAWs - but in two years I will probably spent some money in new hardware (and software ??), so there's time enough to relax and see how things turn out. Jm2C

I've got work to do, so I'm not throwing any towels anywhere. My current expectation is that I'll gradually migrate away, depending on what work is on the table, but I expect to be using Sonar on some near-daily basis for, say, the next two years. Probably using something else side by side within a year.
 
I sometimes work as a writer, and my install of Word is Word 2003. My advice is that everyone should chill out a bit. Are you a Sonar User or an engineer? Are you a Sonar User or a musician? If you're the latter rather than the former, in both of those cases, then there's no big deal here.

I think Sonar is great, and I think the developers made amazing things, and I'm sad that it's over in that sense. But I'm not a Sonar User. I've used Sonar every day for years, but I'm an audio engineer. Cakewalk made me some brilliant tools to use over the yeas. Glad to have had them, sad to see them go. But my towels are freshly laundered every day.

I threw in the towel and switched to S1 for new projects, chiefly because of the OP's point number three.
 
I think the chances of Sonar breaking anytime soon are low. I could probably continue to be proficient from the platform for a few more years. During this time, I would benefit from my (I think this is right) twenty six years of experience with Cakewalk software, since they were MIDI only.
 
During those years, my expertise has grown along with Cakewalk's growth in capabilities. At this point, I can still gain skills using a DAW, but Sonar can no longer grow with me. Furthering my experience with Sonar isn't worth as much as it was before. The investment won't pay off as well, even if the software continues to perform as well as it does today.

I'm in the middle of updating all my backing tracks and this involves using Melodyn to convert audio to midi, Drum replacer to convert real drums to midi and tonnes of work in PVR. I tried Studio 1, Cubase, Mix Craft, Reaper and Mix Bus4.. I bought Mix Bus4 because the price was amazing for what I felt was a solid little audio DAW. Most I felt like I was using Sonar 8.5 and there was way to many things like the smart tool missing. 
I was most optimistic about Cubase as it is at least a mature DAW like Sonar and I have used it on and off for a long time. Mix Craft would have been my second choice I liked some of the midi editing features over all. S1 is over priced compared to Mix Craft.  
 
So I'm not tossing in the towel, but I have back up plans and even if I use Mix Bus from now on for audio recording I'm afraid nothing replaces what I can do with MIDI in Sonar. Not even close. So I think this is the big factor... It is how you use a DAW that will make your decision for you. 

Cactus Music
I'm in the middle of updating all my backing tracks and this involves using Melodyn to convert audio to midi, Drum replacer to convert real drums to midi and tonnes of work in PVR. I tried Studio 1, Cubase, Mix Craft, Reaper and Mix Bus4.. I bought Mix Bus4 because the price was amazing for what I felt was a solid little audio DAW. Most I felt like I was using Sonar 8.5 and there was way to many things like the smart tool missing. 
I was most optimistic about Cubase as it is at least a mature DAW like Sonar and I have used it on and off for a long time. Mix Craft would have been my second choice I liked some of the midi editing features over all. S1 is over priced compared to Mix Craft.

 
The crossgrade for Sonar users to Studio One Pro is $149.00. The Pro Studio version of Mixcraft costs more than that, so I'm wondering why you feel that S1 is overpriced?
 

"If you can keep your head when all about you are losing theirs and blaming it on you..." (first line of "If" by Rudyard Kipling)
 
Some folks like to create crisis where none exists... SONAR isn't going to explode or vanish any time soon, I have X1e->Manchester1 (almost 2 years old now) installed on this machine and they all open fine. They all restore from an image fine. I tend to stay on older things for stability (granted, they have bugs, but at least I know what they are and they do not suddenly change on me). I have kept my home machine on Win7 to keep that stable too... in fact, I just restore a VERY old laptop with XP and that got all of the XP updates (and XP has been "dead" over 2 years now). I have loads of "old" (i.e., 10+ years old) software that continues to run without issues.
 
Planning for the future is a great thing, but do not make things into crises when they do not warrant them (it just causes stress for no reason). This forum is turning into a mob mentality of sorts as a result of things.
 
 

I hope this is not a dumb question, but is there another DAW than can directly and effectively open or import Sonar project files, with project settings, etc. intact?  I assume none will be perfect, but that could reduce the labor a bit.

mettelus
"If you can keep your head when all about you are losing theirs and blaming it on you..." (first line of "If" by Rudyard Kipling)
 
Some folks like to create crisis where none exists... 

If that was directed at me, the OP, I've said several times that staying with SONAR for a good long time is a perfectly valid option, and a perfectly reasonable thing to do.  So no crisis here, though I plan to learn a new DAW as soon as I get to installing it.  For legacy stuff and maybe other things from time to time I plan to keep SONAR installed. 
 
Don't Panic!  It all adds up to 42.

Why can't we just use two towels?