PP is completely right. It's illogical to think Disney would treat offsite guests badly by not giving them the ability to prebook. That doesn't mean offsite guests bring Disney in more money than onsite guests.
They may not be as valuable as a whole to Disney as onsite guests, but that doesn't mean it's logical to think Disney would treat them badly as part of their long term plans. Disney couldn't thrive at the levels it does with onsite guests alone. They need those offsite guests, and offsite guests are theoretically the ones who Disney benefits most from when they prebook. Onsite guests as a whole are already pretty locked into Disney, and didn't necessarily need FP+ like offsite guests do from Disney's perspective.
Who is more likely to buy tickets at the gate vs in advance? Who is less likely to visit Disney every day of their time in Orlando? Who is more likely to make a spur of the moment decision to visit another Orlando attraction? Who has an easier time getting anywhere else in Orlando? Who has an easier time finding cheaper eating and shopping? The answer to all of that is offsite guests. Without giving them the pre booking ability, Disney is basically throwing their hands up and giving up on capturing the extra revenue possible from offsite guests. Giving them the ability to book in advance allows Disney to attempt to keep them on Disney property longer, getting more of their money.
What's illogical, IMO, is thinking Disney wasn't going to go down this road. Anyone thinking past their own "I want this to be a perk for me!" thoughts could easily see it coming.