I think the whole thing about 'any-face' is absurd when taking to its full conclusion. The final point of this reductive argument is the only person anyone can play in any film is themselves - and that's it. I can understand the difference between mockery of a given group of people using stereotypes and lazy performance.... I personally wouldn't mandate against it but I can see why people might get upset - I can't understand policing identity around people whose job it is to change identity. We should be letting actors act.
And yes, Maureen Lipman could've played the role, so could Jamie Lee Curtis, Debra Winger or Bebe Neuwirth but they weren't picked. Likely because Mirren was a better fit for the role and the film has a better chance of success by matching it's characters to the actor's strengths and weaknesses rather than a single facet of their identity.
Great article :o)