No, they are not all opinion based. And sometimes, opinions can be very half-baked and misinformed. So if my opinion is that I am the greatest player of all time, you think that opinion is just as respectable as considering Waldner the greatest player of all time? Really?
But if we were to argue for something a bit tighter like Waldner vs Kong Linghui vs Zhang Jike or Ma Long vs Wang Liqin, then you have to show a sophisticated understanding of table tennis history, maybe for some people some statistical acumen as well, to be taken seriously.
I will be honest that while I don't contribute to those threads, and while I have not liked Carl's arguments, I found yours very unfulfilling. So I decided that rather than go in and strangle everyone while pulling my scalp out, I would just pretend that the threads were like the birds singing in my apartment complex tree... (I am sure many treat my posts that way as well).
Look at the way Larry Hodges handles the argument here (before Ma Long did what he did in 2015 and 2016):
http://tabletenniscoaching.com/node/2037