We've tested open source solutions for s2t, like kaldi, but the quality was not good enough. However, one of the main advantages of a service like assembly.ai to me was that they offer sentence splitting in form of punctuation and speaker detection, which Kaldi does not.
So I guess I answered my own question to some degree: A S2T service is more than just S2T. We already see assembly.ai add more and more features (like summarisation, PID redaction ect.) that are a value-add to plain S2T.
Still, curious to hear what your take on that is.