I think having a TTT while making it essentially mean nothing in GC is a terrible idea. The best part of the TTT is it forces teams to pick one or two rouleur/TT types that they otherwise wouldn't have picked, and therefore being weaker in mountainous terrain, making the race potentially harder to control in the mountains.
Im a bit torn, as I heavily dislike very short TTT's and think they are a meme and useless for the race, but on the other hand don't like plus 40-50 TTT as the advantage is simply to great for the big teams ON TOP OFF already being better off riding for those teams in the other stages. Its quite a conundrum to me, so I guess a TTT around 20-30 km is fine in that it keeps time gaps in check while it still means something.