"make it clean and keep it clean"
Nothing more needs to be said. delay? Extra cost? So damned what.
The flight before got airborne, as did the one after eh? Simple maths therefore suggests a one in three chance of not making it.
Not particularly good odds are they?
Really, there is enough experience and worse, enough buried dead, to make the point perfectly clear. The regulations stipulate it. The SOPs stipulate it. Common sense stipulates it.
Even the 'Bus computers can't change the basic physics of flight.
Contamination = no go.