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.
And yes, Excel still fully supports .xls too.
I fear whatever format LibreOffice uses will die first, case in point I don't even remember what it's called even though I should as a computer nerd.
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.)