Redskins is as offensive as calling a women's soccer team the Las Vegas Bitches or the New Orleans Whores. Whether it offends few or a lot doesn't matter as it's a term used in a derogatory way by white Americans against an entire group of people, people they ended screwing over time after time after time.
While the DC football team may have a long history in American sports, the derogatory, racist, and insulting treatment of native Americans goes back hundreds of years, and as such, that's a trump card the football team can't match.