Why would they even put their troops on the ground?
Because without that, they have nothing at all except the empy threat of witholding gas.
Those type of economic weapons work both ways, and they do so very, very quickly.
There is only one reason that Kosovo was not torn from Serbia in the first place, and that was that the elements of a Russian armored division were on the ground, supplied and in place, at pristina airport.
Serbian forces would be beaten with ease just like they were in the first place.