The OP's ideas are not bad at all, but I would rather they not bother.  Every time I open an old project and get the dreaded missing plugin message, it is a sign to me that I need to stop tinkering around with projects so long that multiple versions of Sonar make the plugin obsolete. In other words, I am dinking around for years and it will never be "done". I now make a habit of freezing tracks so that on the offbeat chance that i want to revisit something i recorded years ago, I have one less issue to deal with.

This has happened to me plenty too  but what i find is that the plugins missing are your usual bread n butter plugins like EQ , verbs delays,comps etc and the mix could be well remixed.
Also the plugins I like, I keep up with their re installs.
I don't often reload old pjt to see if there is something there I 've wasted as am usually creating or re creating new material constantly but it is fun to go back over stuff you might not have bothered to mixdown.
 
For clients the rendering all to audio is a good way to get back what you made for someone if needed.
 

What I find is that the older plugs often sounded bad compared to what is available now.
 
For example, an old synth patch based piano plug in, no longer in the computer.... hey, I'll load up a new plug which uses sampled piano sounds..... waaa laaa, now the song parts sound so much better.....
 
hey no problem for me, let the past remain part of the past.
 
Normally when I listen to the old stuff, it's more of a case of, wow... I could make this sound so much better now with all I have learned since.... but the only question is, do I actually think the song is good enough to spend all the time that would be required to record it all again from scratch....? and the answer  to that is normally  "no", as my writing chops have also improved to the point where I don't wish to spend/waste the time on that old song.

I've actually gone through the trouble of reviewing older projects on my previous XP machine just to make a list of what I no longer have installed.   Unfortunately a good portion of those include soundfont players where the actual soundfont used isn't listed anywhere.   And my oldest projects used all external gear.  Unfortunately I haven't been and still am not a great note taker (hey, I'm sure I'll remember this!) so it makes for interesting times.    
 
Funny though, after going through multiple SONAR version's worth of projects, I haven't done a single thing with them.    Go figure. 
 

I can live with missing effects in old projects, but missing softsynths is more serious.  My biggest headache here has been due to having used the now-defunct DXi versions of several different softsynths, particularly Rapture, Battery 3 and SampleTank.  I would be happy if there was a way of replacing a missing DXi softsynth with the equivalent VSTi version without losing the softsynth's program settings.

thebiglongy
One thing I think people forget is that this isn't "Yesterday" this is "Today" lol. It's nice to reminisce back to the times when you had to just suck it up or search for miles around to find the effects units/mics/synths or whatever if you wanted to rework a project as faithfully (sounds wise) to the original as possible, but with the ability to add more or tweak it up.
But this is today, where we should be able to recall pretty much anything, we shouldn't be hitting stumbling blocks like "plugin not found" just because you've updated your system or the plugins have become outdated and now have superseding versions which sonar won't pick up on....hell, if it did pick up on it, then I'd bet there would be no need for this or many of the other threads relating to that pain in the backside of a message.

Sorry most software works this way, backwards compatibility is often prone with errors simply because it's a nightmare to test and maintain, and sometimes you just have to make a break away from the past. Besides with VST's for instance they are programmed by many developers who do things in different ways (and resources are not unlimited), who work for different organisations.
 
When we have utopia this may be achievable but right now we are living in the beginnings of the virtual synth, complete DAW and effects age, eventually the talent button will be made so we should all think ourselves lucky right now.... Although I'm pretty pissed I haven't got a hover car yet.
 

I too don't mind missing plugs or synths, I always have moved on to something better. I guess this is totally dependent on your music style however. Mine is traditional so I need not worry about some space fart analog synth emulation with just the right tweaks to the envelope filters... So I can see your point in that regaurd..  But I also agree with what a few have said about leaving too many half finished projects around, For me, if I didn't finish it, there was a good reason,,, like it was turning into a turkey anyhow. 
 
And Alex, I too am miffed that we don't yet have a hover car or better yet hover running shoes....what's with that?