in HTCs defense,the OS has to function as smoothly and reliably as stock. we as phone geeks do not care to move apps to sd,change partition sizes,and all sorts of other tricks to get the latest and greatest working on out favorite devices. we will also tolerate some degree of "nonfunctionalitly" temprarily,or even permantly dep on what it is. but,we are in the minority when you look at everyone in the world using htc phones.
there are alot of folks who could care less what OS theyre running,they just want it to work. getting a new OS smooth and polished out for these folks is what really influences wether a device should get an update or not.
ice cold sandwich is an extremely nice rom,but even it likely has some bugs that the general public would complain about. not to mention that if you pushed an AOSP rom to a device previously running sense,alot of folks will complain about the lack of the sense ui,and i have not seen any sense based ICS roms that are close for being a DD for the non rooted masses.
not to mention the fact that htc does not really want you to keep updating your old phone forever. they want you to upgrade and get a new one now and then

the desire for alot of us comes from newer hardware,more memory,faster processors,but for the non-phone-geeks a flashy new operating system that looks different from your old phone could be a good reason as well