There will be some who forgo the upgrade and others who do not. That simple.
If it was me my calculation would go like this: the jump in pay is roughly $3000/month. The bond is $40,000. So it takes me just over a year for the pay increase to meet the bond. So about 14 months after I upgrade I will be financially 'even' if I also resign in that 14th month (actually less, since I believe it is a declining bond - but rough numbers work for this exercise).
So each guy can calculate his breakeven point. It sucks that you essentially work as a captain but get paid as an FO but this is EK!
It won't stop people from leaving though. I have lost count of the number of FO's I have talked to that plan on leaving after getting 500-1000 hours in the left seat!!
There is a yuge (sorry, couldn't resist) number of young FO's hired over the last 6 years. I think EK believed they would come, bow down to the great god that is EK and the big jet, be forever beholden to the opportunity...and stay.
I think EK is sadly mistaken. These young guys are smart enough to see that there is no long term career here and with the hiring going on back home a seniority number is valuable.
There is also the Chinese option: one fellow has planned to upgrade, take off to China and spend 3-4 years saving like a maniac. He is 28, will be home in the UK with a fully paid house and can then earn less but be at home. Not a bad plan if it works for you!
Until there are no options or EK makes adjustments they will bleed pilots.