Why Doran's shield needs ALWAYS to be an underwhelming option

Because it's a deffensive option. That means that if at least one of the oposing laners buys it, the laning phase becomes stagnant and uninteractive, or - IN THE WORSE CASE - their power is equal and then every trade shifts it's favor to the Shield's owner.

In scenario where Doran's shield becomes as good as offensive Doran options, there would be no reason to build them other than to hurt yourself. We can see it got a bit buffed and is already becoming a problem, not to mention the abusers like Vlad, which fills the second example above already.

Thank you for your attention.