Let me clarify for you what I mean by 'death sentence'. Time frames can vary but it is not uncommon to follow this career path:
Enter EY as 320 FO. 2 years later get 330 CCQ. Remain 320/330 FO for another 2 years. Upgrade to Captain - go back to 320 single fleet. 2 years later get 330 CCQ. Remain 320/330 PIC for another 2 years. Now move to 350 (maybe). Do the math: that's 8 years consecutive on the 320. Gaining a 320/300 qualification is not an 'upgrade'. Remember you can be 320/330 and fly the 330 once every 2 months. Somge get lucky, some don't. Having the 320 on your licence is like a disease that won't go away and keeps kicking you in the ass.