I can understand the point of view, but I kinda think it's a bit short-sighted to look at it in that way. When you're done with the sequel, and there won't be more, if you want to play the game again, you are forced to either play with the old content, or the new. Wouldn't you rather play a new game with everything thus far, rather than have to go and play each half of the content individually again? I rarely play games through twice, for similar reasons, but I'm always hoping that the developers would try to make games with as high replayability value as possible. Replayability equals longer life, which equals better or at least prolonged sales, which in turn increases chances of even more additional games/content.
And technically, it would be possible to make a sequel that goes together with the original, effectively being an expansion. For example, you could allow the player to choose whether to play on the new continents, or the original... or jump between them. Those that only want a new story and new maps can choose to only play through the new continents. Developers could balance the experience income and everything to allow full leveling with the new continents alone... while still allowing those that want to experience 'the best of both worlds' to choose to play with the old content simultaneously as well.
This all is just my point of view, of course. No offense meant to anyone.
