I think that's basically why some mileage programs which allow you to directly purchase miles limit how many miles you can buy and credit to a given FFP account each year. Otherwise, nobody would pay for super-expensive full fare coach tickets, they'd buy enough miles for a non-saver award ticket (no blackouts or capacity-control limitations). Yes, there are other ways to acquire lots and lots of miles, but they require more effort, whereas simply buying the miles directly from the airline is so straightforward that they couldn't allow this loophole to stand.