When 5.7 is released, I hope the the (new?) marketplace will add a new "sort by" filter for "5.7 Compatible". I think everyone will see the value and agree its important enough to bring up now. Maybe this is already planned?

I think perhaps it will have a secondary benefit (beyond the obvious) of giving add-on and theme developers even more of an incentive if you will to update their products, maybe sooner than later.