The logical structure of this argument is flawed. Here's another argument that shares the same structure, but is clearly wrong:

If you owned any slave and could cheaply do so, you'd want to ensure it doesn't die of neglect. But should you treat your future selves as your slaves?

Here's another version that makes more sense:

If you had an opportunity to mold a friend to share exactly your preferences, and could do so cheaply, you might still not want to do so, and wouldn't be considered irrational for it. So why should you be considered irrational for not molding your future selves to share exactly your preferences?

One answer here might be that changing your friend's preferences is a wrong because it hurts him according to his current preferences. Doing the same to your future selves isn't wrong because they don't exist yet. But I think Robin's moral philosophy says that we should respect the preferences of nonexistent people, so his position seems consistent with that.