15 000 PC's is a lot but remember Microsoft are adding 20 000 SharePoint users day. Something that sits well with Windows Server and does everything you have outlined already and has done for years. Google are not that injudicious that they would take this fight to Microsoft and IBM.
ChromeOS will never work, unless it works like Android, which Google does not want it to, so it will never succeed. I would stop using computers altogether if my OS was run by Google.