On some level, yes. However, there is not enough room in overhead bins for a legal-sized rollaboard for every coach passenger. Therefore, much as a I dislike it as a fairly tall person (though not 6'4"), I support flight attendants insisting that bags that fit under the seat go under the seat.

Delta's allowed width of a rollaboard is 14". For three passengers per row, that's 42". There are only about 31" per row. There's a bit less overhead bin space than 31" per row because some overhead bins are reserved for crew/safety equipment, and the edges of the bins reduce the practically-usable space. (This is on a 3x3 configuration; MD80s are of course even worse.)

So if everyone takes the "allotted" space in the overhead bin, about 1/3 of the passengers will have to gate check a bag. The only way around this is the new bins that Alaska has on some planes which allow rollaboards to be stowed on their side. If everyone keeps the expansion closed so rollaboards are only the allowed 9" tall, that does mathematically leave enough space for everyone to stow a rollaboard. (Getting people to actually stow their rollaboards on their side is another issue; even getting people to stow their rollaboards wheels out or wheels in is enough of a problem!)