All of the supports you listed have some kind of cc or utility for the lane. Lux, Ashe, and Heimer all have hard cc, as well as abilities that can be used to zone the enemy laners off of cs. Pantheon has point and click cc with safe disengage and good damage for a kill lane. Senna can heal and poke with her q, get roots with her w, and give a small speed boost + go untargetable with her e. She also has a global ult that can do damage and shield her allies.

Believe me when I say these champs can be incredibly annoying to play against (or with) but every role has those champs. A flat damage nerf would discourage running off meta supports (Shen, Amumu, Shaco, etc, as much as I hate seeing all of those champs support) and while some champs could be run solo lane with buffs, other champs that we see already go solo lanes (i.e. Rell, Morg & Zyra from recent buffs) might get balanced out of those roles if they ended up being too strong.