They don’t, if you join as DEC you’d have the exact same fleet/trps/holidays/pay as the guy next to you who’s been there 10 years. The only difference is depending on the base a farther car park and less loyalty bonus (at least!). STILL I haven’t seen a (non retired) CX captain join any of these companies in the last 20 years.
I know of 2 CNs who left CX for DEC at Jet2 in the past 2 years