Of course, if you use per/Mb pricing, then those services suddenly become much more expensive.

Which means that any competitor to those services now faces a high barrier to entry - they can't just compete on features, they have to be that much better to overcome the cheaper cost of mobile data for the product they're trying to compete with. Which leads to easier formation of monopolies.

Which is exactly the kind of thing that was pointed out as the problem requiring Net Neutrality in the first place.