I think there's a reason Uber chose cutoffs around 4.6 (depending on the city) and I don't think it's because they consider someone with a 4.5 to be a terrible

I think there's a reason Uber chose cutoffs around 4.6 (depending on the city) and I don't think it's because they consider someone with a 4.5 to be a terrible driver. Uber faces a similar issue to taxis: the supply of potential drivers is too high. Too many drivers means each driver can't get enough fares. Cities solved this for taxis with the medallion system. Uber could limit drivers arbitrarily by preventing new sign ups, but if their competitors don't do the same, this puts them at a disadvantage without solving the problem: the drivers they prevent from signing up will just go to Lyft. Using a star rating as a cutoff is smarter. The drivers with the highest star ratings are the ones the riders generally liked best. If Uber's competitors want to take the lower rated drivers that Uber rejected, that's fine with Uber. Riders will start to find, on average, that Uber has higher quality drivers, giving Uber an edge over competitors. The 4.6 minimum rating may have been tuned so that there's enough drivers that riders aren't kept waiting, but not so many that each driver can't get enough fares.