Other systems don't offer the performance that Colossus offers, is why. POSIX has all kinds of silly features that aren't really necessary for every use case. Throwing away things like atomic writes by multiple writers allows the whole system to just go faster.
It sounds like you have to find a design that meets your performance target and usage patterns - just like anything else. It also sounds like Google's CFS is a grass is greener situation - you heard Google had something that solved the problem you have, so you want it. But the reason it sounds good, compared to the other designs, is that you haven't had to actually use it and run into its quirks yet.
Google's internal systems have been written against the Colossus semantics for many, many years and thus benefit from it's upsides (performance, cost efficiency, reliability, strong isolation for a multi tenant system, ability to scale byte and IO usage fairly independently, tremendously good abstraction against and automation of underlying physical maintenance, etc) while not really having too much of an issue with any of the conscious trade-offs (like no random writes).
On the other hand, if you've been building your applications against expectations of different semantics (like POSIX), retrofitting this into your existing application is really hard, and potentially awkward. This is (IMO) why there hasn't been an overtly Colossus based Google Cloud offering previously. (Though it's well publicized that both Persistent Disk and GCS use Colossus in their implementation.)
One of the reasons why it would be extremely hard to just set up or build CFS elsewhere or on a different abstraction level is that while it may look quite achievable to implement the high level architecture, there is vast complexity in the practical implementation side. The tremendous user isolation it affords for an MT system, the resilience it has against various types of failures and high throughput planned maintenance, the specialization it and its dependencies have to use specific hardware optimally.
(I work on Google storage part time, I am not a Colossus developer.)
