I think the management answer to this is that you can just hire 9 offshore not-seniors for less money, and that somehow equals the experience of a single onshore senior. In that sense it's a kind of the experience equivalent of the 9 women/1month == baby analogy, and obviously wrong. Unless you're in management.

More or less correct. It's not that management believes in "9 offshore juniors", it's just that they don't know (nor care to confirm) who's actually working for them at the outsourcing firm.

The combined incentive of cost cutting at the outsourcing firm and foolish MBAs in the west opting for the cheapest outsourcing means that the offshore does actually employ juniors, who do build up the experience to become seniors.