Well, the thing is that the game can be completed without expanding the worlds. The advantage in the expandable worlds being optional is that it allows for a more varied play experience as well as allowing people to complete the game in different ways based on their skills and frustrations.
The way they've made it sound is that the game can be completed via completing the base worlds, and can be expanded with Pagies. However, they've noted that the world expansions contain within them even more Pagies. Thus it's theoretically possible that the game could be completed with a player barely touching a particular world due to the option of expansion. If a player, for example, is too much of a novice/rusty and/or finds, let's say, an underwater level too frustrating to play, they can focus on the worlds they enjoy and are more comfortable playing to complete the game, and then will maybe come back to the "shunned" level to try again with more experience.
I'm excited for expandable levels, and I think the fact that they're optional really allows for a better gameplay experience for everybody. For the old school collect-a-thon thugs like myself, we'll happily toss our pagies to expand our worlds, even if they're not our favorite. That being said, a newer player or somebody who cares less for the exploration and collecting can skip the expandable worlds and have a good time without feeling bogged down by integers of items.