>

Hi, historical linguist here. Yes, words and their meanings do change. I do understand your point that mute, mad, lame have all been stripped on their offensive meaning and that you believe that retarded has as well. And to be fair, it might have for you. But language change is not a solitary process. Language is a social tool used by, shared by, and shaped by all of its speakers. 
The word retarded grew to incredible levels of offensive and taboo in the early 2000s. The folks for whom it became taboo are in their 30s-40s and their younger siblings are in their 20s. This  became a formative taboo for them when they were children. They will likely hold onto that for a good, long time. 
The thing with mute, dumb, mad, and lame is that they are all much older, and their phases of being taboos was much further in the past. There's a natural cycle for these things:
word with specific meaning > negative connotation of meaning > used as a slur/in an incredible hateful way > word becomes taboo > enough time has passed that the term loses it's taboo and either returns to the specific meaning or because of slur and taboo stages, has changed in meaning enough that it has adopted a new form, such as being a general marker of negativity (e.g. dumb and lame). 
I think you're right that it is moving towards being like those words -- but it's a strongly engrained taboo. That takes a serious amount of time to change. It might not be until millennials and zoomers are quite old, or are gone, that this term stops being a taboo.
/u/MrNincompoopy (OP) has awarded 3 delta(s) in this post.
All comments that earned deltas (from OP or other users) are listed here, in /r/DeltaLog.
Please note that a change of view doesn't necessarily mean a reversal, or that the conversation has ended.
^Delta System Explained ^| ^Deltaboards