I have significant doubts about this “any Tesla as robotaxi” idea at least if you’re referring to Musk’s empty promises about how you could make money off your own Tesla while not using it.
The simple explanation is indeed that they’ve noticed 2 seats is enough for most taxi usage and IIRC I’ve seen some articles say the data supports that. They’ve optimized the Cybercab to be their taxi in this and other ways. Clearly they’ve also made efforts to maximize its range and chosen an interior that is easy to clean, removed the glass roof and even the rear window that would possibly not last so long in heavy use etc.
If there’s ever going to be a bigger taxi from them I’d expect it to be another purpose-built vehicle.