Microsoft Excel has a web-based version and Google Sheets is pretty good. So unless your enterprise needs spreadsheet features that those don't have, it's fine.
Which are: VSTO plugins, pivot tables, equation editing, database connectivity, import-from-file, document protection, ... The list goes on. Excel on web is not at feature parity with Excel on desktop.
The real answer that already exists to "having a thin client with native Excel" is to stand up a Windows Server instance with Excel installed and a Citrix gateway, and have your thin clients connect to it. My bank does it.
The kinds of people who rely on Excel to do their job assure me that Google Sheets is not even remotely sufficient and the web-based Excel isn't quite there either.
