"Never" is a long word. Leylandii will develop new shoots from old wood if dosed with a high nitrogen fertiliser. I've regenerated one Leyladii hedge that had been allowed to shoot up to 20ft by cutting it down to six ft. Then getting inside and cutting a V shaped section along the length of the hedge to allow light to penetrate into the interior. Then followed five years of weaving in braches, cutting back to let light get to the developing shoots and the regular trimming it should have had from day one. The next owner of the property kept up the treatment after I told him what we were doing and today it's a fine hedge. As good as a typical yew hedge IMO. But it has taken near 15 years to get it that way.