Yes, building trams instead of two different systems seems like a good idea. But just have in mind, that unsufficient separation from other transit modes leads to problems like car accidents, congestions, lower reliability etc. of both modes. I can show you a map of tram network in Prague (I live there), which shows, where there is separated ROW for trams and where the trams ride in mixed traffic. In general, we have exactly what you've described. Mixed traffic in the center and separated ROWs in the outskirts - trams with separated ROW were primarily built after 1930. (see map below). But we're constantly trying to get rid of those red places. It's sometimes slow process and even in city with majority people travelling by mass transit there is a car lobby (they think they defend "freedom to drive anywhere by car" at the expense of lot of other people taking mass transit) , but we have got a lot of successes behind us in separating tram (and even bus lately) ROW and calming/diverting car traffic, where it's not possible to separate it. With separated ROW it's also much easier to make a priority for trams/buses at crossroads with lights. (We have 70% traffic lights with tram priority)
But I know, you've got much different situation here (but I have to pinpoint, that Prague has almost the same population, as Columbus urban area), but even then I actually wish, that America had more inspiration from for example succesful French tram systems, which are newly built and are in fact really a combination of light rail and streetcars. They fulfill primary function of mass transit => to transport people between places, but they also bring important change to urban environment and spur development and regeneration. To do both purposes, they have to be separated from traffic on most of places, so that they are faster, than bus they replace. French actually think (and we in Prague too), that lowering number of lanes because of tram doesn't do any harm, because lot of people previously driving or going by bus will still go there but they'll take a tram, so no bussinesses will be harmed.
I think, that it's not a disadvantage for High street in Columbus, that's it's already well developed. But streetcar/light rail/tram have to bring some new quallity, otherwise it's pointless to exchange mixed traffic buses for mixed traffic streetcars. From what I've seen, High street is wide enough almost everwhere to retain parking on both sides, one lane in each direction for cars and there still can be separated ROW for streetcar in the middle of the road. So no need to eradicate cars because of streetcar. But I really don't get, why you built all the highways crossing all Columbus, if you didn't want to calm traffic in densely populated places like High street a little bit - make it more human.
Explanation of Prague tram network map:
Dark green - separated ROW, Light green - mixed traffic without any problems - low car intensities, yellow - mixed traffic, where there are sometimes problems with parked cars and/or congestions from time to time, red - mixed traffic - there are often congestions severely innfluencing tram system reliability
sorry if it's too offtopic :-)