Ah yes, the old Denver trolleys.  While that was before my time here (I came here in the 60's), I've read about them. We used to go to the Old Spaghetti Factory downtown quite a bit which is in the old central trolley building.

Of course, those ran down valuable street space which was (and still is) far more important for automobile use once it came around.  Most of the RTD tracks have been located off to the side with their own right-of-ways, so they're likely to stay in place much longer.  (Although unfortunately they did include a number of street-stealing stretches of track too downtown, and that new stupid stretch in Aurora.)

You're point is valid though... it's always possible something much better will come along in the future that could make those tracks obsolete, like the automobile did for the old trolleys.