With business class products continuing to improve overall and elite benefits being slowly whittled away, I expect fewer instances of elites being able to access a higher-end lounge than Business Class. There are definitely times where it is understandable why (Oneworld Emerald, for example, can access the first class lounge when flying BA JFK-LHR, even on a $400 'we're matching Norwegian' eco fare, while a $10k last minute business class passenger cannot).

Polaris Lounges are continuing that trend which many other airlines already do, as you mentioned. I believe AA is changing their Flagship Lounges to allow business class passengers as well, rather than just OWE and F pax.