On any decent size project real senior programmer / architect with years of active product design and development under the belt would beat the crap out of the person who learned to screw size 8 bolt into size 8 nut and been doing this and nothing else for years. And seniority does not come from knowing every last function/class of some framework.
Personally I've never had problem doing project in new languages and had never have client making a point out of it. They just look at the references and products I've completed personally or as the lead. But those clients are business with real needs and problems to solve. Not architecture astronauts who would go to holy wars over things that do not matter much in real world.
On one side you pit a "real senior programmer /architect with years of active product design and development under the belt" and on the other side "person who learned to screw size 8 bolt into size 8 nut and been doing this and nothing else for years".
My point is that for this situation:
* person A (truly senior polyglot developer just now starting to use a specific stack)
* person B (truly senior developer with experience primarily in that specific stack, all other things being equal)
Person B will smoke person A at the start of the project, since they're already ramped up. Person A will have to learn, that's why we call it "ramping up".
Personally I've never had problem doing project in new languages and had never have client making a point out of it. They just look at the references and products I've completed personally or as the lead. But those clients are business with real needs and problems to solve. Not architecture astronauts who would go to holy wars over things that do not matter much in real world.