They'll pay, and they'll pay with a smile on their faces once the alternative is paying or there being a 75% tariff as opposed to a 40% tariff on everything coming to the US from Mexico.
It's all a matter of what the alternatives are and so far, the alternatives have been to do nothing now or to do nothing later.
If the not so well off and uneducated are stuck in Mexico then Mexico knows they will have a revolution there, thus they want to send them here while they enjoy their lavish lifestyle.