Thank you for your feedback. For now, you can use an API through Powershell to import .mpp files in Project for the web. You can learn more here: https://techcommunity.microsoft.com/t5/project-blog/import-project-desktop-files-to-project-for-the-web-api/ba-p/2031298
We are also looking into other ways to support import. Stay tuned for future updates.
An error occurred while saving the commentSimon commented
I signed up for the Project online trial and lost interest the second I saw you can't do something as simple as File->Open. The mind boggles.