There is a huge space of possible games to play between those two ends. B/X out of the box and 5th edition out of the box are not our only options. There are vast numbers of things that you can like about either game and want to have in your campaign, without loving everything about either.With games like Basic Fantasy and Old School Essentials available, there's no need to try to change 5E to be more old school. These games are cheap/free, easily accessible for anyone with experience with 5E, easy to teach for completely new players, well supported with tons of adventures and supplements and are just fantastic games.
When you add more and more elements of one to the other, then at some point it might become reasonable to consider switching to the other one as the baseline and modifying that to your taste. But even between that hypothetical halfway point and 5th edition out of the box there's still a vast range of possibilities to run your campaign.
How would you port over warlocks, bards, and druids with all the land circles ,and exhaustion over to OSE? I think at that point it does become sensible to consider 5th edition as the baseline and tweaking that.
And that doesn't even go into the subject of player interest.