"as to how many would leave EK if DEC recruitment is started again"
So why do you think anyone would leave? If EK upgrades 150 a year and gets 10-20 DEC:s (they can't get more, I promise you) it would have a very marginal effect on the time for upgrade. And, if you have waited three years in EK, would you leave just "to make a point" (that no one would care about) for another carrier where it will take you another 10 years?
DEC:s will not create cheers but I doubt people will leave because of it. DEC:s or not, there's simply no better options than EK if you're looking for a widebody command and the FO:s know this.