"tens of millions"? you must specify what. hosting mail with 20GiB average mailboxes? Or a publicly accessible file share with a million monthly hits? Big difference. I would not productionise Nextcloud for a large organisation.
Finally there are very few orgs worldwide even with a few hundred k employees and any firm would get huge Microsoft discounts at that scale for products like SharePoint (on prem), Exchange (on prem) and Office Server (on prem also)... and oodles of support over a very mature product which Nextcloud is simply not.
Finally there are very few orgs worldwide even with a few hundred k employees and any firm would get huge Microsoft discounts at that scale for products like SharePoint (on prem), Exchange (on prem) and Office Server (on prem also)... and oodles of support over a very mature product which Nextcloud is simply not.