There are many reasons why (at the moment) I would prefer classic URLs by default. The Blue Ocean UI does not cover all our uses of Jenkins yet and I'm not sure it ever will (due to different vision). It's mainly enabled to track the progress and see when it becomes useful for us. I disabled it when all the "build failed" emails sent our developers to the Blue Ocean URLs.
Most of our projects are either freestyle or matrix-style projects. We are very used to the project page showing us at a glance the build status of the last changes + some trend graphs that show the evolution of build times, test runs, code style checking, build warnings, .... At the moment only the build status is available in the "activity" page. The other 2 pages ("branches", "pull requests") are not available at all due to not using multibranch pipeline. So, we converted one of the matrix projects to a multibranch pipeline and occasionally check how that is displayed in Blue Ocean. The branches tab works there, but as we are not using github (not even git), the pull requests tab will stay useless. Nowhere in Blue Ocean do we get the trend graphs we are used to in Classic.
So, I understand we are not the target audience for Blue Ocean at the moment and that's fine with me. I only have it enabled because I am interested in Blue Ocean and want to track its progress. As we're a small team, I'll just ask all our developers to change the URL setting themselves, but being able to set the default would have been preferred.