...it appears that the UHF remote has gone the way of the dodo...
Agree with your other points, but not this one. All of the dual-tuner receivers come with dual remotes, one of them UHF since it's meant to control the receiver from a remote location. And also (not sure about this) all of the PVRs come with UHF remotes. (My 501 does.) So UHF remotes are very much alive and well at Dish Network.
OK, now I'll turn the question around and answer the opposite. Reasons you might want to get a newer receiver include the nicer on-screen guide, PVR functionality, HD reception, dual-tuners, reduced antenna cabling needs (1 coax/satellite), and greater maximum cable length (now 200'). Or some combination of these improvements.
Let me add comments about just two of these features. The nicer on-screen guide includes a sort of picture-in-picture, with 1/4 of the screen showing live video from whatever channel you're on, while the rest shows the guide. This is also very handy for domestic harmony since you can look around for better programming while your spouse continues to view her stupid

home and garden show.
Also, (and correct me if I'm wrong about this), dual-tuner receivers incur only a single $5/month additional receiver fee. This would save you $5 over your current setup.