I think the current style is doing the game a favor. When I look at Civ IV screenshots, I see a style that tries to reach for realism in the same way that other games like AoE did, which is using more toned down colors and generic proportions. To a certain extent, the pixelated image kind of makes it appealing, but otherwise, it lacks charm.
Basically, the overworld style in VI has a certain quality that Civ IV or V didn't have. The trees are regrettable, but I can see it improved if they make forests less sparse. It will certainly look better than the green blob of rubble in CiV. And yes, CiV forests and jungles didn't appeal to me, even if they do have subtle differences between continents.
Also, people who praise CiV like its style is the apex of realism probably can't explain why when my units walk into forests, they look like they are wading in a sea of bush. I don't see why CiV should be excused for making the units giants against the Lego-block cities and the mossy forests, if realism matters that much in a game like Civilization. At least Civ VI tells us from the get go with its distinctive look that gameplay purposes trump realism any day in a strategy game.