follow-up to this:
rooting the phone and installing a custom ROM (liquidsmooth) on it was really not that painful at all. The thing now runs Android 4.4.2. I basically followed this guide.
It's even faster than before (no wonder, since all the Motorola bloatware is gone) and the battery seems to last longer. There's a few bugs, though - it's not as stable as it used to be with the stock Motorola software but hangs every now and then (once or twice a day, especially when in GSM mode). And the camera doesn't work anymore.
But all in all, this proved to be a cheap and fairly easy way of extending the life of the device another year or three. This is my travel phone, and does not get heavily used - just voice calls, SMS, a few travel apps and some minimal internet browsing and google maps navigating for ~$8 a month in fees.