So what, exactly, are the major translation sites you are talking about? The only other site I know of that could be called “major” in terms of volume is dhammatalks.org. If we expand the definition, then ancient-buddhist-texts.net would be included. And if we really stretched you might include suttafriends.org.
I don’t see any incentive for those three sites to publish their translations in two places. It’s a headache they don’t need. Especially when it comes to making any corrections or updates. They would have to do it in two places.
Also, for folks not aware, there are two kinds of translations on SuttaCentral. For the user the difference is kind of hidden. One is the “legacy” translations. These are hard coded html. They do not have the ability to show line by line Pali with the translations. The license on these translations varies by the author.
The second type is the “Bilara” translations that are done using the Bilara software. This facilitates the creation of translations in a segmented fashion, allowing them to be (at the will of the end user) displayed line by line with the Pali. The license on these is CC0, similar to public domain.
I find that Bhante Sujato is really open to suggestions on how to make SC better. But I also feel people need to respect when he has made a policy decision on how the site will run. I wouldn’t want him to have to spend time defending that decision when it could be spent doing productive things on the site. I don’t think it has ever been the mission of SC to provide all translations available. So your proposal would mean a change of that mission.
I believe (and I could be wrong) that if any of those three sites wanted to add their translations using the Bilara system they would be welcome to. That seems reasonable to me. But it would require them to change their license.
I think you know about the Citation Helper tool I created. Using that will cover most of the translations available free online. Since I created it I have been watching the RSS feeds of the sites I mentioned above and there have been no new translations added to their sites. I have also written up instructions for using a browser plugin so you can select a citation on a web page and instantly open it in the CitationHelper showing all the translations.