OTS needlessly adds the requirement that the user publicize their .ots
files to everybody who will make use of the timestamp.
Publication is not a component of the OTS system.
This does not provide the service you describe. It would be trivial to
include enough cryptographic information in the original OP_RETURN, so
as to obviate the need for publicizing the .ots file.
(Why would it be needless to require everyone to publish OTS files but not needless to require everyone to publish via OP_RETURN? In fact, now you have blockchain users that don't ever use your OP_RETURN data.)
If I send my .ots file to another party, a 4th party can replace it
with their own, because there is no cryptographic pinning ensuring its
contents. This changes the timestamp to one later, no longer proving
the earliness of the data.
You can't replace a timestamp in the OTS system; you can only make a new timestamp. To use the earlier timestamp, you would have to use the earlier timestamp. At any time it is allowed to make a new timestamp based on the current clock. The use case for OTS is proving document existence as of a certain time and that if you had doctored a file then said doctoring was no later than the earliest timestamp that can be provided.
I was just talking about this the other day actually...
https://news.ycombinator.com/item?id=31640752