Well this characterization isn't quite right. With regard to TP2, this is a solved problem, with or without using a server. Most of the conversations around TP2 seems to miss the crucial point that TP2 is not needed for convergence. CRDT academic literature has largely been responsible for propagating the notion that TP2 is a must for every OT system - without TP2, all is lost -- this is a myth. I wrote about it extensively here .
This article  gives an analysis of 7 coeditors based on OT, including the likes of Google Docs, showing that they avoid having to design transformation functions that require TP2. Some of the systems are fully de-centralized , others require a server.
It is also worth making a finer distinction between systems that do need a central server -- there are "transformation servers" and "messaging severs". Google Docs, for example, demands that its servers perform some portion of the transformation logic. Many other solutions rely on using the server only for message transmission/serialization, thus the server side logic is significantly simpler.
 Achieving convergence in operational transformation: conditions, mechanisms, and systems.ACM CSCW (2014)
 A generic operation transformation scheme for consistency
maintenance in real-time cooperative editing systems ACM GROUP (1997)