If the enchanter support is what is keeping the ADC alive, isn't more logical to kill them first?

People already do this with Soraka. They focus Soraka because they know that if Soraka is kept alive, she makes her allies outsustain them and win trades or even teamfights with her healing. Plus, the less health Soraka has, the less she can heal her allies, since she needs to use her own health to use her heal.

Why the same doesn't apply to other supports like Janna or Lulu? After all they are the reason why the ADC is kept alive. Peharps it may be better as an assassin or burst mage to kill the support first instead of the ADC? It may sound like a bad idea given that you are not taking out their main source of damage, but think of that way. You are an assassin and you kill their support. Now, your mage and your ADC can wreak havoc because they have lost the main thing keeping their ADC alive. Most hypercarry ADCs rely a lot on their enchanter support to be kept alive. And if the enchanter support use their shields and heals on themselves to keep them alive, that means free reign for the rest of your team to kill the enemy ADC.

Another reason why I was thinking it may be better to focus the support is because most enchanter supports are very squishy themselves and they, in theory, should be easy prey for the assassin, and the only reason why nobody focus them is because people already have in their heads the idea that focusing the support is an extremely bad idea unless it is a Soraka.

In this meta, peharps an assassin's role is no longer to kill the enemy squishies, but to open holes for the actual carries on their team to allow them to deal the most damage possible.