If Discovery does end up a failure, then they should just go post nemesis.
Most fans seem to want post nemesis over any other premise offered for consideration, at least going by polls conducted here that I have seen.
Going for post nemesis would be the best way of retaining existing fans and adding more to the fanbase.
If Discovery fails, it's not going to fail because of the time period it's set in. It will fail because people simply won't want to watch a new Star Trek show. Setting it post Nemesis will not change that.