Because that is the point of balance: utterly gut a champion because they are not fun to play against, without any real compensation or rework until said champion is so bad that actually picking them becomes a liability for your team in almost every situation. It worked out real well for Olaf and Evelynn who sat in the toilet for years before finally being worked on again when some guy at Riot scrolled through their champ list bored one day "Oh yea, I forgot we have this champion in the game. Maybe we should take the time now to rework them."
Why not do it from the get-go? If an ability is so cancerous that people are whining and moaning about it, remove it and give the champion some real power. As far as it stands come next patch Tahm will be so abhorrently bad. He will NOT be a support tank. He will be a top lane cheese pick who people will notice does far less damage than he used to. He will have no purpose in team fights, no power to secure kills, and arguably not even that tanky even with a second freaking health bar.
Mark my words, his win rate will drop below 40%.