If you get promoted and are earning less than everybody else doing the same job, you are PAYING FOR YOUR TRAINING ( in capitals twice for the slower among us )
Sorry, that argument just doesn't hold water. When I became an F/O on the 767 I was on considerably less than an F/O (doing the same job in your terms) who had been in QF for 11 years. Even when I checked out on the 744 I was still on year 11 pay earning a little less than those on year 12 pay. Was I paying for my training? Most F/Os on the 767 are on different rates of pay. Most S/Os in the company are on different rates of pay.
Whilst I'm always one for change and a radical overhaul of the pay system to redistribute things towards a more equitable outcome (F/Os on the 767 work longer and harder than any S/O on the B744 and should be remunerated accordingly....spare me the arguments folks, I've seen both aircraft), you can't just throw lines like yours out there cunning and not expect to get smacked down.