Quick tell MA they ruined Martin Jones for giving him 19 starts his first NHL taste of the NHL and never went back!
What's that, both ways work as every player is different? Who knew!
Err, well, you did say:
It's how most good teams actually evaluate goaltenders.
Except that's not how it works in the majority of cases. Here's the progression for ten of the league's starters:
Braden Holtby- 14 and 7 NHL GP over two seasons before becoming backup
Jonathan Quick- 3 NHL GP before becoming backup
Petr Mrazek- 2 and 9 NHL GP over three seasons before becoming backup
Ben Bishop- 6, 7, 13, 10 and 9 NHL GP before becoming backup
Cory Schneider- 2 and 8 NHL GP before becoming backup
Devan Dubnyk- 19 GP before becoming backup
Craig Anderson- 6 NHL GP before becoming backup
Ryan Miller- 3 and 15 NHL GP before becoming backup
Jake Allen- 15 NHL GP before becoming backup
Jaroslav Halak- 6 and 16 NHL GP before becoming backup
The ones missing from this list either become a starter or a backup immediately upon playing their first NHL game; since we know that wasn't and isn't the case with Brossoit, these are the best historical comparisons available.
In each instance, anywhere from 3 to 45 GP were required before their respective NHL clubs deemed them ready for second-string durty; while you were correct in that all players are different, it's extremely rare that a team anoints their backup without first auditioning them ten or more GP- often spread over multiple seasons.
Chiarelli, however, has apparently decided he's smarter than history. Our previous management groups were often criticized for believing they were bold mavericks with genius schemes (see: Swarm, The; Kotalik and O'Sullivan). I'm not sure why Chia is above reproach in this regard seeing as he's about to make the same sort of mistake.