Russia's air force is a top 3 air force when it comes to quantity and quality (at least on paper) and for over a year now the nation they are at war with has been penetrating Russian air space but this latest one is inexcusable. This latest attack was done by more than one cessna aircraft which have the RCS of a barn. Where was the Rusian air force? Why during wartime knowing their airspace is getting penetrated is there no CAP? There's two answers to this, sheer incompetence or they don't have the capability to secure their airspace from their neighbor they are at war with.
These aircraft came from within and during peace time if US was at war with Mexico the US would have 24/7 CAP with AWACS especially if they one time penetrated US airspace and struck a target in the US.
My point is that you can never have 100% coverage and 100% interception. You are just dreaming if you think otherwise. In any case, it's not like Su-24s are flying into Russia and lobbing Storm Shadows at Moscow.
If 50 Cessna-drones got through and killed Putin and all the senior government officials along with blowing up the Winter Palace and the Kremlin, then we can say it's inexcusable. 3 getting through becomes a carefully planned attack.
You say "RCS size of a barn", but a Cessna is a small aircraft, it is hard to detect. This is why civilian aircraft normally carry transponders, makes it easier and more precise to track.
You've also invalidated your own excuse. Your logic is a prior attack should be a reason for heightened precautions. Do US presidents still ride in convertibles just because JFK was killed in peacetime?
Putting it another way, why do you think it is "Russians don't know what they are doing" rather than "Ukrainians know what they are doing"?