The economics of a megaship probably don't work as well for Disney since it's a premium price point relative to others and they don't lean hard into speciality dining upselling, gambling, etc. Having a 3,000 capacity ship full during slow season and pulling in huge rates during busy seasons "maintains" their market position more than a megaship that's full during busy seasons and discounted during slow times.
They're also probably scared to write a check big enough to build a megaship... they would probably 1.5-2X the cost of the Icon if they tried to build something that big.