Also sending any hostile aircraft near to Kyiv now means hello Patriots. Very different kettle of fish to 15 months ago.
I am not sure Kyiv in 3 days was a good plan 15 months ago, but it wasn't that far off working. It was probably a 50/50 in all reality.
Its probably a 99% chance of failure now. I believe from last autumn onwards that area has been mined to hell, its difficult terrain anyway, and will have nicely zerod in arty etc.
I doubt they'd have taken Kyiv quickly - in a city like Kyiv even a few thousand fighters with prepared defences/stockpiles would have taken substantially larger forces to defeat than Russia had committed. Even today you need armies of the scale of WW2 to effectively siege somewhere like that.