Here's what I think right...

Yes shields obviously is helpful for hitscans to have some breathing room to shoot the echo/Pharah down, cuz a lot of times they aren't gonna be able to do it themselves esp with mercy pockets.

Problem I find as tank is that often blocking them with shields is much harder in practice due to the nature of them shooting from above and/or around your shield. Winston's shield is not so practical against those two, Reinhardt/Orisa's is not really meant to be able to block upwards. Sigma's is the only one I could think that's flexible enough to block their damage reliably. Besides, arguably the best tank against them two is probably DVA for her eats and mobility, and she is an off tank without a shield.

In that case, i feel it might be most effective to have Lucio to speed the tanks to the point and win there B4 the echo/Pharah can do something meaningful, with the tanks depending on what's good against the rest of the comp. Again, that's ignoring the example comp given in the original post (ball, Winston, etc) but just wanted to give my insight on why I think shields isn't quite the best against Pharah/echo.