This is an interesting point and you could also mention Ruri James whom I've seen do really well and very convincingly as Tony.I have not yet seen Matthew Seadon-Young as Tony, but I have seen Robbie Durham perform the role a few times, and I must say....... I can't understand why he was not given the role full time!
Down through the years at BETM there have been quite strong Ensemble members/Alternates/Understudies who were never promoted to Principal Cast. I'm not sure but I suspect this may be the norm in theatre but would be interested to hear the views of people who are more au fait with the subject.
There have been some who made it. Those who spring to mind are Kate Graham who took over the fulltime role of Mrs. W for quite some time when Joanna Riding went on maternity leave almost before she started. Kate was quite magnificent in the role possessing probably the best singing voice of all the Mrs. W's I have seen. She is Mezzo Soprano (described as Eb below middle C to super D). When she was in the Ensemble her vocal accompaniment with the Miner's in Once We Were Kings was stunning. She was from Easington so the Geordie accent was natural and an Irish accent was in her portfolio as well . She went on to be in Charlie and the Chocolate Factory.
Phil Snowden who, in his first stint in BETM covered just about every male character including Dad, Mr. B and George is, of course, now Principal Mr. B.
David Muscat was in the Ensemble in the Australian show and also covered Mr. B and George. When he came to London he was again in the Ensemble and alternate Mr. B, the Principal role of which he subsequently took over.
Off the top of my head some who didn't make it all the way despite delivering super performances as alternates were Mrs. W's, Caroline Deverill and Gillian Kirkpatrick, Tim Funnell and Stevie Hutchinson(Tony's). I'm sure there are more that others may recall.
There must be a fine line drawn when choices are made but I'd also believe that, like all other professions, "who you know" comes into it.
Pat