Enjoyment is much more important than many managers think. Attrition is real, especially nowadays. Enjoying the work, which includes the programming language among other things, plays a big part in hiring and attrition.
And with regards to employee retention, I would rather bet big on benefits (paid time off, parental leave, etc.), reasonable working hours (4 days per week), achievable goals, great communication, democracy and good team cohesion. I have a feeling this works for a lot of people. If a language is the only thing that makes you stay, by all means find a place that truly makes you happy in that regard. All I am saying is that there are a lot more variables to worker satisfaction.
Those are important factors also. But I would prefer to work 5 days per week and enjoy my work rather than working 4 days and not enjoying it because the PL sucks. Just my 2 cents.
Yeah, I would like to see a source on that.