Except that it's proprietary and will eventually be un-maintained and stop working. But spreadsheets, in general, fall under the Lindy effect and open source software will continue it for centuries to come.
MS-OOXML is barely an open format. Have you tried implementing it? ECMA-376 part 1 is over 5000 pages, and there are four parts to it! (Part 1 contains an extra bit about SpreadsheetML, but by that point we were two zips deep. I turned back ere I got lost.)
The OpenDocument formats, meanwhile, are older, simpler and better than their MS-OOXML equivalents. (The ODF spec is 1041 pages altogether – 215 pages of that are the spreadsheet formula language.) LibreOffice's implementation is a little janky, sure, but I can edit OpenDocument files by hand. Try doing that to a MS-OOXML file. (Good luck.)
Its easier for me to imagine a world in which AI makes spreadsheets invisible to the modern person than it is to imagine a world in which Excel isn't the de facto spreadsheet.
