There is still only speculation that this will be the last version. That said, it will most likely fit the bill for a lot of apps in it's current form. If SL5 can give you what you want, I don't see a real problem not to use it to develop your next app. It has the tools now and it's in production now.
If you don't think an app you start writing today will still be being used in 10 years time, you're being short-sighted in my opinion. That kind of attitude is the reason so many companies are still stuck on IE6 (which came out just over ten years ago) due to poorly written intranet apps.
You raise a point which is interesting if there was an official word from microsoft that this will be the last version. Right now this is still only speculation. Anyway, from a pragmatic view point SL5 is available today. A fresh release like this is a good reason to rejoice instead of speculating when the next release will be. Out of curiousity, what alternate are you suggesting that has the same unmatched tooling, documentation and support "today"? Specifically, if the mobile/tablet is not your target and your main audience is desktop on mac/windows and perhaps linux(note: with a small effort you can make your app work on moonlight too).
Edit: note also that with a small effort you can target the new windows 8 marketspace and release it as a metro style app as well.