I think you're letting the allure of the money downplay the costs of actually doing the work. It's not like one client is going to hand you a duffel bag full of $100 bills, you do the conversion once, and then you're set for life. It's going to be a seemingly endless line of enterprise clients who already have a history of not keeping up with the times.

The amount these clients pay, while substantial, probably won't provide you with the "fuck you"-money needed to never have to do it again.