If science is statements about facts, then philosophy is statements about other statements, the only requirement being that these statements are non-contradictory.

That makes mathematics just a subdivision in philosophy, in which statements about statements must be (axiomatically) derived exclusively from a completely explicit starting point. Since math mostly took over logic and epistemology (as computability), general philosophy has indeed lost its most important subjects.

Philosophy is simply meant to shrink and downsize. Reasoning without any particular method gets naturally replaced by reasoning with one. If philosophy had grown, that would not have been a good sign. It would not have been progress.