Squishy supports as a whole will never be top tier because...

We already have squishy mid laners and squishy adcs as the standard for their roles. You can only have so many squishies that need protection before a team comp becomes unviable and mid and adc already take up 2 out of a possible 5 spots in the vast majority of games. A squishy support at that point is just another person who needs protected; and the only squishy supports that are actually good are ones that do a very good job of protecting themselves or are so strong that fighting entirely around them is a viable strategy . Champions like can be extremely strong on disproportionately tanky teams, but you see those so regularly that they just seem bad in general.