I think many get way too hung up on the "future" of mobile devices.
Look, the phone you choose now is what you will have for a year or two.
If it works, NOW, and can achieve the things you want, NOW, then who cares about next year, or the year after - when that time comes, you will decide THEN what to do.
So, to answer the OP's question, relative to the Pre 3, here it is, in a nutshell:
Those who really likes WebOS, but wanted a bigger screen, a faster CPU, a camera with more pixels and autofocus, and are on ATT or T-Mobile or any worldwide GSM carrier, guess what?
The ultimate WebOS device, to date, is available for you to buy, at affordable prices. There are tons of apps for it too, both on Preware and the HP app catalog - more than enough to last you the next year or so.
No phone will last us forever, sadly enough. With LTE coming, and quadcore processors emerging next year, once again, smartphones will evolve, and any device you purchase today will be completely obsolete next year...
The key here is, TODAY, to buy the device that fits you and what you need, and want, and should be able to last you for 1 - 2 years.
My Pre 3 is an incredible fit for me; its not the fastest, the "koolest", the most "speced out" device, but, it seems to be doing everything that I, as a WebOS user, who needs a smartphone with multitasking and multimedia capabilities, want and need, with incredible hardware that should last the next year or two, not to mention the homebrew community here, that will, if I continue to support them, help me keep my device customized and performing in top condiition, as well.
So, goals achievled, for me; now, onto other things in life that need my attention.