This practice has been a significant ancillary revenue maker to EK for decades. Only the most obliged will pay cash upgrade on board (I have done this few times in the distant past, but stopped almost a decade ago after realising a paid J/F fare was almost always cheaper than a Y upgraded onboard).

As a general rule, if the flight is overnight then do whatever it takes to escape Y.

Also upgrades ex-DXB are often much pricier than outstations (quoted $1.5k J-F for a 3 hour flight, whereas inbound was like $900 with similar loads but YMMV).