"I wonder if gun makers would bother making 7 round magazines."
Companies are in business to make a profit. What would be better for them than having millions of their previous products made useless and a new market for millions of new product created with the stroke of a pen. I'd bet some of the larger high capacity magazine makers are already jumping up and down for joy and having preliminary meetings to discuss the new gold rush for them. Magazines do not take the level of re-tooling for manufacturing as does items more complex.