Wayland provide "forced solution" for that problem, that does not mean X can't have some solution to prevent tearing .
And which known dowsides that solution has?
Wayland force users to use compositor, xserver does not .
Some people have issues looking at anything which is composited, so Wayland is not something which magically "God send to as" .
An image can be composed by pixel coming from different source or as a part of a single source, but at the end it's an image sent to the screen. Y frame for each second, but still they are images sent to the screen. What kind of differences the people see (or think to see) between a composited screen vs a not composited one?
It is just software (althorough with good ideas), but nowhere near finished nor so widely supported and it is nowhere near robustness of X, which is this article all about .
About the "finished" and "supported" points, the things can change pretty much in just 6 months, we will see.
About the robustness, and I add the security, a slim protocol is better to fix and maintain rather than a giant mess like X.