It was there in 1979 when I went down for freshman orientation at Fort Lewis, and it was very new.
People might bitch (as we've seen with the fire, people in Durango are quite vocal in their opinions), but the owners of the restaurant can do what they want. I'm sure the city would encourage them to be mindful of where they are, but it is not an historic district, so there is only so much they can say.
As an aside, while I was in school there, (1984 grad) that McDonalds was a test restaurant for many of the new items. This was before Al Gore invented the old interweb, and the only TV Durango got at that time was out of Albuquerque. (Why many old time Durango residents are Dallas Cowboy fans, rather than Broncos fans. Dallas is who they got on TV). They trotted out the McRib my freshman year (never liked it), the McSalad, (which went nowhere), the green shake for St Paddy's and several other fails that I can't remember this many years later. The thinking was it was an isolated community both geographically and media-wise, so they could get a read on the products in a vacuum.