Very interesting, but I am not totally convinced yet.
About point 6, "exceptionally hard to linearize history", shouldn't simply looking at timestamps work well enough?
About points 19-22 I think it is to be expected that people will be able to use the service as a storage for their files, that is something people can always do when there is a public forum/whatever allowing files.
Main question: if not matrix, then what do you propose to use instead of matrix?
@dashall@hachyderm.io