Yeah, I'm feeling this.
I'm in a similar pickle right now ... I really like my HTC Desire S as an overall package, and the things driving me to upgrade are, for once, external factors. Rather than doing it for the techno-joy of gaining new features or shoving a terrible operating system for the promise of a better one, or grabbing the best ad-hoc replacement I can because the previous phone was broken/stolen, it's more the matter of replacing something that's good, but a bit worn out and no longer able to entirely cut the mustard. Like an old laptop once it's been upgraded to the hilt, or an otherwise perfectly good car which has failed its MOT and can't be economically repaired.
The screen's cracked (but I can deal with that, and would have had it replaced by now if I couldn't), the casing is all scratched and worn and bruised (battlescars, m'boy!), the battery is shot (that's a £10 replacement, really), the mSD card is a bit pokey (I could still double its size if I wanted, and take a more rigorous approach to spring cleaning)... but moreover the OS is out of date and can't be upgraded, which means I can't connect through authenticated proxy servers and is even starting to affect app compatibility, the CPU isn't quite strong enough to deal with 2014's multitasking requirements (it was fine in 2011), and the internal RAM and Flash is horridly overloaded. All of those things in combination spell "may as well replace it".
But the form factor, the sheer usability of the OS, the ruggedness, and the ease of switching battery/card/SIM (and repairing genuinely broken bits) are all just fine by my standards. The camera could be better, but it's not a critical point. And any connectivity problems I've had have always been down to signal strength rather than download speed, so I couldn't give a stuff about 4G / LTE / Wimax or whatever unless they meant I was more able to get a reliable connection, rather than a faster but still patchy one, in places with borderline RF field density.
What I'd really like to do is have something just the same as it was when I bought it, but with refreshed internals. Scads more working memory and system flash, faster dual core CPU, newer OS, and optionally a better camera and more pixels on the screen (and, if it can be arranged, greater touch precision so I can use a really thin-tipped stylus and have at least as much control over the result as I had with my trusty old Palm IIIe). Keep everything else as is.
Seems that the "give me a Desire S with updated circuitry and firmware" scores a big "hahaha no" on the "what manufacturers want to offer the customer" front right now though. Bit of a problem. Every single handset misses the mark in one way or another. It's either too big ... or too primitive / not actually advanced in any measurable way ... or too limited (locked-in battery, no card slot...) ... or has shockingly poor battery life right out of the box ... or is goppingly expensive (I got the DS *free*, on an £18pcm 24-month contract...) ... or uses a horrible OS (I want Android, so I can bring my apps and cloud data with me) ... etc, etc, etc.
What to do, other than buying something with a hopefully small enough PCB and doing a frankenstein job to wedge it in the existing frame?