First of all, let me remind you about Time Machine. The TUAW crisis response team all agreed that your backup issues could well be met by that simple OS X utility. (TUAW commenter John B responds, "Time Machine and iPhoto libraries are not close personal friends," stating that Time Machine won't do small differentials, instead "it'll want to backup that entire ~100 GB iPhoto '11 library all over again vs. just the bits that changed". A quick TUAW google of the matter seems to suggest otherwise, but we're happy to hear more from commenters.)
As for the upgrade issue, Auntie turned over your question to the team to see what they had to say on the topic.
Uncle Mike, author of an upcoming Apress iPhoto book, told Auntie that he has never understood why people say iPhoto can't handle over 10k photos. He writes, "My iPhoto has 19,800 photos in my library, and it runs just fine. Well, it ran slow as a dog when I only had 2 GB of RAM in my MBP, but now with 6 GB of RAM the program runs as fluidly as Mail does." Mike also added that iPhoto really does need to make having multiple libraries easier for the user.
Other members of the team preferred to move to Aperture. "When you have more than 10k photos," one wrote, "iPhoto can't really handle it well. Aperture can do multiple libraries making life even easier."
Certainly, the new lower cost of Aperture at the Mac App Store make it an inviting alternative. As another team member points out, maybe with our new digital lifestyle and iOS cameras, Apple is finally getting how many photos people really take. Maybe the Aperture repricing is to respond to that new demand for bigger, more extensive libraries.
If iPhoto would simply do multiple libraries more gracefully than it does now, it would probably be fine for you. But it is currently as graceful as an elephant peeling a hard-boiled egg.
Love & hugs,