I think they do have to go with mega ships, actually. Holland America has nothing to do on board.
DCL and HAL are largely not vying for the same customers. And there simply isn't enough room with the smaller ships to add premium attractions.
That said, you can make a mega ship without putting 8,000 people on it, that's certainly possible. You could remove one or two decks of cabins to reduce capacity, but you're still going to have a long boat with a wide beam. DCL is looking at the types of attractions Royal is launching on Icon, and they know their customers are going to expect multiple big experiences aboard.
I expect the future ships will be more like a modernized version of the Disney Adventure (I say "modernized" here because even though it's a brand new ship, the Adventure is based off a fairly old ship design at this point), and the Adventure is going to have a rollercoaster onboard in addition to an AquaMouse type of watercoaster! I think the new ships might even have more to offer in that regard.