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Aren't pianists who play Bach becoming fewer?
Literal
Pianists [genitive-の] Bach-players [subject-が] is-decreasing [nominalizer-のではないか].
A musing in question form. バッハ弾き ('Bach-player') is a compound using 弾き (the noun form of 弾く, 'to pluck/play a string instrument') as a suffix meaning 'someone who plays X.' Productive: ピアノ弾き ('piano player'), ギター弾き ('guitar player'), そろばん弾き ('abacus user'). The ~のではないか ending is a characteristic Japanese rhetorical device: literally 'isn't it (the case) that X?' but functionally 'I suspect X,' 'perhaps X is the case.' It softens an assertion into a tentative proposal for the listener to consider — common in essayistic and analytical prose.