There really aren't very few of us, but most manufacturers see the keyboard as a gimmick. Most people don't care about it, but if folks who WANT QWERTY keyboards are still buying other phones, they're fine. I think probably 20% of the Android market (which is a lot!) care about having a QWERTY slider -- but of those people, 90% of them will buy a non-slider phone if necessary.
That being said, I think the next "evolution" of QWERTY's will be phone manufacturer's building bluetooth keyboard cases. I spoke with a few rep's for Motorola, and they said they would consider building a bluetooth keyboard case for the X-Phone if there was enough interest. Want a keyboard? Tell Motorola their original Droid line rocked -- and that we want a bluetooth keyboard case for the X Phone. Chances are, they're going to keep the same "phone" size/shape going forward for next year's X Phone's, so I can see us buying a "keyboard case" once for them and using it for several iterations.
I've contemplated the same thing -- could I just physically build a keyboard or 3D model a keyboard case that could be used on a standard bluetooth keyboard, and allow a user to 3D print (or pay me to 3D print) the enclosure/case for their phone? It'd be a lucrative business, no doubt, for someone to come up with a "standard" keyboard for Android devices and then allow folks to use that keyboard with different phones by providing different cases that would fit the phone + keyboard.
Per the Relay 4G. The camera sensor is supposedly better, but it doesn't have the same resolution depth... but I rarely go over 2 megapixels anyway. It also has a 5-row keyboard, from what I can remember and the processor is a bit faster (about 50%, I believe). The Captivate Glide is analogous to a Samsung Galaxy S 2.3 or so -- it's weird and kind of in between a 2 and 3. The Relay 4G is very firmly a Galay S 3. Better battery life as well, due to it being a Snapdragon processor.
I will probably be purchasing a Relay 4G in a few months so that I can switch to T-Mobile. I'd like to keep my Captivate Glide, because I have a feeling next year, we'll see some better QWERTY phones. Will they be GSM -- that is the question.