I bought my X50/520 back in December of 2004 and it's still running like new. I haven't developed the need for anything particularly more powerful yet. The screen's still fine as is the performance.
As for why the PDA market hasn't yielded faster performing units - I'll venture a guess. I'd be willing to bet that the technology behind the power (batteries) has reached a bit of a stall right now. To put faster processors in these boxes you would shorten the battery life. OK ... the newer processors might also be lower power per MHz - but it still might be more of a power drain to double the performance of a handheld. If my experience is worth anything - nothing I'm doing on my PPC is much of a limitation when it comes to speed. The manufacturers may have realized this - so instead of making them higher-performance as they are they're cramming phones into them. Once that has happened I wouldn't be surprised to see higher performance chips follow - but only once the battery technology can deal with it. Who wants a PPC/Phone that has a battery life of 2 hours?
I'm also very un-inclined to want a Phone/PDA combination. I'd rather my phone be just that - my phone. It's not very often that the functionality of those two devices winds up overlapping in my case with the exception of a contact's phone number or something.
My thinking here is along the same lines as someone who posted earlier, I think. In my particular case the technology is reaching a leveling-off point. I've already got more capability than I'm using so - unless I'm replacing my unit why would I want to upgrade? I.E. unless I'm just hot to get some new gadget I'm not motivated to fix a problem I don't have. Same thing with Vista ... but that's another thread.
To answer the original question: "Is the X51V 'long in the tooth'?". I wouldn't say so. I think a device that you have can only be described that way if it's about to quit working or has somehow become inadequate for what you need it for. I have a hammer that's 20 years old sitting in my garage. It could be described as "long in the tooth" I guess - but it hasn't broken and it still drives nails as well as it did 20 years ago. About all that's "long in the tooth" in that regard is ME!
Just my 2 cents.
-CB