I confess I'm not as fond of joint crews/troops in many cases. Either this happens, or da crew program is shortchanged and becomes just Older Boy Scouts. Stand-alone crews seem to work OK, but then what we're findin' across da country is that they don't have staying power, eh? They tend to fold after the founders age out.
That having been said, I know a couple of programs that run joint troop / crews that work very, very well. They're runnin' more on da international scouting model, where crew members are also leaders of troop patrols and help run troop (or sometimes Webelos-level cub) programs. Da PLC and the crew overlap quite a bit.
Those programs don't have any problems maintaining older boys in the troop, and the kids seem to handle the extra commitment just fine. The crew is giving 'em fun and adventure with friends, plus a lot of learnin' new skills. The troop is giving 'em scope to use those skills with their friends to lead and teach others. They've got very full calendars, but the youth are the ones workin' the calendar and they seem to cope just fine. It's gettin' the adults to keep up that's more challengin'!
My advice for your Scoutmaster would be that if older kids are leavin' the troop program that's on him, eh? The troop program isn't providin' enough scope for their talents. Rather than try to discourage 'em from joining the crew (which never works), encourage 'em to take what they learn in the crew and use it to lead the troop. They should be the ones runnin' mini-high-adventure outings for the troop, leading instruction for the troop, managing safety and drivin' the troop program to new heights.
When you've got kids like that, da role of the adults is to get out of the way as fast as yeh can. It's being treated like responsible adults whose skills really matter that keeps boys in the troop.