Business is business. If a publisher is refraining from going into ebooks altogether because of Amazon, when they get the option to direct sell and also access the other ebook platforms as well, I feel they're not long for this world either way.
It's difficult to prove a negative. Ebooks are a growing market in the US, the EU and SEA (though TBH I have no idea about China amd India)
It's been more than a decade that it's mainstream and takes about 10~20% of book sales depending on the country.
Amwzon's profits are still split with the publishers and publishers have their own venues + competitors (in particular outside of the US) so I'm not sure why Amazon's presence is a blow against the industry as a whole.
What more would be needed to see it as a validated business area?
Ebook sales have trended down in the US since 2013[0]. They got a pandemic bump in 2020 which didn’t put them over the peak years in 2013/14 and like many things had a correction downwards afterwards[1].
Amazon maintains 70% of the US ebook marketplace and keeps a much bigger portion of the money from books published through their publishing arm than they do book published through the traditional publishers. They also have less high standards for publishing. This makes it harder to find traditional published books on the biggest platform for selling them.
I'll take your point: there seems to be a US specific problem with ebooks.
In particular, the lack of numbers from the dominant player (Amazon) makes trend analysis all the more difficult. Yet there's still an increase after 2020:
On Amazon, are you merging publishing and distribution as a single activity ? I thought Amazon in the US only accepted self-publication under their name and the rest of the Kindle available titles are from each publishers only using Amazon as a printer/distributor.
E.g. who would you assign as 50 shades of grey's publisher for the ebook version?
Amazon has both a self publishing arm (kindle direct) and a traditional publishing arm (amazon publishing). Like most traditional publishers they have a variety of imprints (e.g. 47North for Scifi/fantasy). They compete for authors via the same mechanisms other publishers do and have notable non-self published authors (Greg Bear jumps out to me as a scifi fan).
I don't know the specific legal contracts involved with 50 Shades of Grey's ebook distribution but I'd assume that Vintage Books bought those rights after they became the publisher for the book.
