you're right, i have in fact met someone who thinks dumb, stupid, etc. are all slurs and should not be said! (i am from a pretty liberal place, so bound to happen). i personally think the "history" of words should never be a criterion for slurs, just the way it is used today. so if people think the r word is primarily a slur today, i'll comply, but i think dumb and stupid aren't even about people half the time ("that was so dumb", "this game is so stupid"), so calling these slurs would be bad.
a story about computer architecture: they used to use "master"/"slave" to discuss a particular type of interaction between machines. but recently, they have switched to "master"/"worker" because of the connotation of "slave", which i can understand. but a class i took decided to switch to "controller"/"worker" because even the word "master" has connotations of slavery, which i don't understand - i don't care if it originated from slavery, 99% of the time today it is used for other things (mastery of something, kinky reasons) and it's lost its old connotation.
i guess my pov is, i haven't done huge research into the usage of any slurs, so i have to listen to what society tells me. and since the r word has very easy alternatives, i have no issue trying to switch to other ones.