Why. Use your imagination instead of being a sheep that can't walk and chew gum at the same time.
Here's one way to make it so it doesn't fuck up people with self shields with added effects to them. Make it so the stronger shield will override the weaker one EXCEPT for one point of health to the previous shield. This means that Sion's shield, if it wasn't the strongest shield would stay active until both the more powerful shield and Sion's now 1 HP shield got removed so he could still use it as he needs for the damage.
There? Happy? Or would you like Riot to look at Reddit where they were suggesting things like this?
What about diminishing returns for shields? Subsequent shields only shield for 1/2 of their value within a certain time the first one was used.
Like the debuff summoner heal has.
I think the solution is some kind of shield breaker and/or preventer that's gated by a long cooldown. If you make it too easy to break shields, then champions that are balanced around strong shielding get deleted straight out of the meta. But what we want is something strategic that offers both a solution to shields and a way to resist that solution for the shielders to stay relevant.
So for example, put an active on an item that substantially reduces the effects of shields, or instantly deletes shields, or some combination of those effects for X seconds. Put it on a long enough cooldown then you get one chance to use it per teamfight.
Zenbuster:
3200 Gold
350 health
40mr/40armor
10% cdr
Build path: Ohmwrecker + mantel + gold (debatable path)
Active: Zenbuster active: Prevents nearby enemy towers from attacking for 3 seconds . This effect cannot be used against the same tower more than once every 8 seconds. Additionally when activated, nearby (1500 range) enemy shields take double damage and incoming enemy shields have reduced effectiveness by 40% + (2%per level) effectiveness for 6 seconds. This effect cannot apply to the same enemies more than once in 12 seconds (120 second cooldown).
So yeah, fuck me for trying to come up with something reasonable that can be worked with and tweaked instead of a freaking EMP bomb that just remove all shields, which would equally fuck over one of my main champions, Tahm, if not more so than Sion given how Tahm's shield works.