But by definition if a company grows for decades it doesn't 'exit' (=it isn't sold or renamed or repurposed). So I think that the Mittelstand definition should prohibit an exit, maybe not for the individual (=founder/owner) but for the company as a whole. In my opinion this is why in german you don't really use the term for companies anyway except when econonomists look back and analyze your company history.
I begin to think that OP just took the word "Mittelstand" to mean "$10M-$1B", which at least to several other people in the thread is rather confusing.
From there, they will have many exit options, including corporates, PE, and even IPO.
I don't believe there's anything in the Mittelstand definition that prohibits exits.