'plonking' meant without consideration for corruption effects. If you have the continent to yourself, then you are presumably managing the city distribution to minimize, where possible corruption. One obvious tactic being not to ICS, since that hits against the OCN limit faster for a given land area.
Any city with 21+ base shields is going to be producing at least 2 unwasted shields; the 95% cap will ensure that. Depending on the location you might be better rushing a factory and a plant, rather than the courthouse/policestation combo - it all depends if the C+PS will get you below 95%. If they wont, you can use the fact that 50% of 2 is 1, and 50% of 1 is also 1, so the city would then have 4 shields. (I agree, thats still fairly pitiful, but it's something). If you force the city into WLTKD, you can probably unwaste another shield, at least.
And dont forget you can generate all the science and tax revenue from specialists, reagrdless of corruption. So that totally corrupt 2 shield city may be capable of a decent science or tax rate contribution. (If I had a 30 shield city at 95% corruption, I'd abandon working 9 of the shields, and use those citizens as science or taxmen)
If you've got such a huge empire that both FP and Palace can't provide enough anti-corruption, then surely if there were no corruption (or much less) it would just reduce the end of the game to a pseudo-milking exercise. Your massive territorial strength would translate directly into output and the AI would be squished like a proverbial bug.
Corruption provides a way to handicap the stronger players, by identifying their strength through the size of their civ, and using that to key the corruption. Reducing it will make life no easier for the weaker players, while reducing the challenge to the strong.
Obviously this is very much a personal taste thing; it's clear that some think its too much and others dont. I agree its frustrating to conquer a bunch of territory, only to find its not as useful as you had wished or expected. But the game design decision appears sound, to me.