I believe Balpa members would prefer a handful of fixed term contract Training Captains over DEC's. As you say - it will be training capacity and not SFO numbers that will likely be the issue.
I only know of three Captains ever asked to leave/fired and they were all DEC's. It's like Forest Gumps chocolates - you never (really) know what you're going to get despite the best efforts of the recruitment team. Whereas SFO's with 4+ years in the company are a fairly well tasted chocolate and unlikey to turn into an Orange Creme in the LHS...
DEC's won't happen.
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