There are occasional hiccups in 8.1, but those are largely due to it not having the newest firmware yet that is set up for 8.1. The Developer Preview program is for enthusiasts, but the firmware for the OS release is not included. It still runs fine, but none of the tweaks are there. I expect the firmware release will come with improved speed, etc. That said, it's snappy as it is. All Windows Phone 8 devices are capable of running 8.1. I expect all of them will run as fast or faster as they did with 8. Word was that Windows Phone 7 devices could run WP8 faster than WP7, but that the hardware encryption featured in WP8 is the reason they could not be upgraded. That wouldn't shock me. Microsoft has done something pretty crazy on the desktop side of things these past five years - each successive iteration of Windows has improved speed, reliability, and features while DECREASING requirements. Windows 7 needed less than Vista and Windows 8 could run on less than Windows 7. I suspect that they are constantly tweaking WP8.
I'm sure there will be a time when some devices will not move forward - like there are cut-offs with Apple as well. Yet I've seen 8.1 running on Lumia 520/521 handsets with only 512MB of memory and it is buttery smooth. Cortana works well even on those low end handsets that you can buy for $50 or so with no contract.
The 928 would not be as future proof as the Icon, but I'd guess it will be able to at least move up to Windows Phone 9 next year (if that's what Threshold is called). That is likely to be the merger of RT and WP. We're supposed to be getting the ability to connect peripherals like printers, hard drives, etc to our phones in the next year via the micro-USB port and that should come to all phones too.
I think Microsoft has done an incredible job in creating a modular kernel. The sophisticated software engineering often goes unnoticed nowadays, but they've managed to fit the NT kernel into handheld phones and make it work quickly and effectively. It's not really a dumbed down form like it is with iOS and Android in relation to various UNIX versions. The power is already there in the 928 handsets and other WP handsets. Microsoft only needs to unlock it.
Just conjecture here, but I think they are really aiming for OneDrive to become THE cross platform data repository. I suspect they will open it up to allow ALL apps on ALL platforms to store data there, allowing for easy appdata transfer between platforms. My guess is that MS hopes to facilitate platform agnosticism and then create the best platforms for doing different activities - making it easy for people to move their way, but also doing it in such a way that they don't feel backed into a corner.
I think the 928 should be safe for two years. The Icon should be safer and does have some nice features. That camera is killer, but the 928 has optical image stabilization and a backlit sensor. It also has an incredible mic system that the Icon improves on, but that doesn't make the 928 less impressive. It's not 1080p, but it is 768p and has a very good pixel density. It's not quad-core, but Windows Phone really gets along fine with dual core. I had my 822 for a year and a half as my daily driver (same processor specs as the 928, just different camera and screen res) and never suffered from the lag I've seen happen over time on Android devices. It may not hurt to occasionally reset the phone, but Windows Phone 8.1 saves most app settings, your Start screen, texts, contacts, etc. My Start screen came over from my 822 without a hitch and all my apps automatically redownloaded. If you reset and leave your phone on wifi for an hour, you'd probably not notice the difference between before and after.