if they keep to their usual cycle, the a5 processor won't get ios9 - the a4 didn't get ios8 so it's the a5 next. but there's a glimmer of hope, as ios9 is said to be more of a performance update (think osx snow leopard)
Snow Leopard wasn't a performance update, it was an optimization and new technologies update. It was an overhaul of the infrastructure of OS X. If iOS follows suit here, then it is highly likely that legacy support in most areas will be dropped and that includes A5 support, which, at this point has required extra effort to maintain in updates to iOS 8.
It would stand to reason that from a development standpoint, they are incentivized to drop support for the A5 as soon as they can.
From a marketing standpoint, the iPod line, iPod touch included, is declining and it hasn't received updates. Apple has the freedom to drop support and discontinue it and, relative to their customer base at large, it wouldn't harm them in the slightest.
The first generation iPad mini competes with perfectly adequate Android tablets that will never get iOS updates but are still more than functional as tablets. You get what you pay for. For them to finally drop support for it wouldn't be all that unrealistic. People forget that even though the A5 has found its way into a lot of devices, most people with iPhone 4Ses have upgraded as have iPad 2 users. Those who are still rocking a first gen iPad mini would hardly care that they're not being kept up to date as they're buying a tablet that was released two and a half years ago, also the mini tablet market is dying in and of itself.
Long story short, anyone who would care that their A5 device is old has already done something about it or at least has a plan to do something about it.
so if i was to buy a brand new itouch 5g today, what life expectancy could i realistically get from it?
That depends:
1. As a music player, it will last until the critical components die; and if you take really good care of it and its battery, you should get easily 7 years before any one component starts to die.
2. For E-mail, provided your e-mail will still work with whatever version of Apple Mail is the final one for it, same amount of time.
3. For apps that connect to a service, you have until the final version of the app that supports the final version of iOS that runs on the iPod touch is no longer able to connect to said service.
4. For apps that don't connect to a service (games and whatnot) you can theoretically run the last compatible version of those apps until your iPod touch finally dies.
5. For as a video player (for iTunes content), see 1.
6. For browsing the web, it will last until the browser no longer supports the majority of the Internet.
7. For the iTunes store and purchasing media, in theory it will last until Apple no longer supports those devices from the store (which likely won't ever happen for iTunes and iBooks at least) or until 1. happens, whichever comes first.
8. For being current on iOS, odds are decent that you have until this September until it is left behind. I'd imagine that if you didn't load up that many apps, the relative lack of support for security updates wouldn't affect your ability or experience in playing back media a la a traditional iPod. I'm considering buying a 64GB fourth generation iPod touch from the refurb section of the Apple Online Store just to use it as a media player and see how I like it out of the context of it having been an iOS device (as we often forget that iOS was originally billed to deliver the superior iPod experience before apps were ever a factor).