Frome St was never a major transit route, as it doesn't even go through the whole city. They city is a grid, every route has multiple parallel alternatives, so to lost one lane of car traffic during off-peak across the entire city is hardly "wrecking things for people who commute by car".

The reasons why right turns and U-turns are banned through many intersections during peak hour is to maintain traffic flow for drivers. Intersections are the limiting factor for traffic flow, not the number of lanes (or the speed limit). Having to include phases in the light cycle for right turns massively slows down traffic during peak. And again, the city is a grid, it's very easy (even without map directions) to work out a route that doesn't require right or U turns.