Yes, but the 5 is still the 5 regardless of it being "positionless." We still need someone who can play in the post, and have some height on defense. So while the 1-4 (and especially the 1-3) are completely "interchangeable," the 5 is still somewhat of a true position. So sure, while EJ would be a huge get for us b/c of his ability to play the post as well as on the perimeter, we still need guys who can play the somewhat traditional 5 spot. Conditt and Castleton are perfect examples of guys who as the mature can probably move out and play the "4" spot as well due to their ability to shoot and handle the ball. This helps make taking both of them not completely out of the question.
Defensively I hear you. We need height, length, strength and rim protection from the 5. In a power 5 conference that defensive combination is essentially non-negotiable.
However, offensively, ideally it is truly positionless. That is what made the SFA teams so good. Their "5s" would change between jacob parker (6'8), their point guards (5'10-6'0), thomas walkup (6'4), and a giant 6'8 football player looking guy whose name I can't remember. They didn't play one at a time, so at any point any of them were in the high post, others outside. Your concerns about post play offensively are legitimate, but something else to consider is that BU will have our guards going through post up drills. On the other side, not a lot of guards are used to guarding the post, so our guards will be comfortable there, while their defense might not be. Should the defense switch to keep a big guarding the post, there will be other mismatches to take advantage through counters and in the basic flow.