LibreOffice has never been an excellent alternative, IMO. It comes across as a mediocre port of a copy of Excel from 15 years ago.
Teaching users who are not great with computers and who know modern excel how to use some 1999-era LibreOffice Calc software is not fun. LibreOffice recreates the worst parts of menu/modal hell and it's not a place the modern user is good at navigating.
In every case I recommend Google Sheets over Calc these days. So much easier to teach, more intuitive design, and more simply interoperable with the outside world.
I have occasionally tried open office and clones for a very long time and "adequate alternative" was the best impression I ever got and far from the worst.
So, by any reasonable definition, Oracle's (and now Apache's) version is the fork, and LibreOffice is the main line.
LibreOffice is a continuation of the original development team, and the original code. The only thing they don't have is the name, which doesn't matter much in my mind. So why do you say that what's now called "OpenOffice" is the main line, and "LibreOffice" is a fork? What's your definition of a fork, exactly?
Teaching users who are not great with computers and who know modern excel how to use some 1999-era LibreOffice Calc software is not fun. LibreOffice recreates the worst parts of menu/modal hell and it's not a place the modern user is good at navigating.
In every case I recommend Google Sheets over Calc these days. So much easier to teach, more intuitive design, and more simply interoperable with the outside world.