Well, Kearny there is arguing that it would be "incompetence or treason" to not attack a strongly fortified city with two corps' worth of defenders while having no valid supply route."Which shows that Kearny was a military incompetent."
Pretty strong statement. I think it shows he was frustrated over the snail like advance up the Peninsula in general. He was still angry over the supposed sleight he and Hooker were given after Williamsburg. Kearny led from the front, unlike McClellan, and was a hard nosed aggressive officer who believed in fighting. Perhaps if a little of that had rubbed off on McClellan...
In war, logistics matters, and so do fortifications. With supplies and good artillery support then getting into Richmond in a couple of weeks is quite possible; without supplies and trying to do it in a couple of days is just going to exhaust the half of the Army of the Potomac that hasn't just broken at Gaines Mill, leading to the loss of the entire Union army.
If fortifications manned by 40,000+ men could be overcome so easily by troops with rough numerical parity then we'd have seen it happen at other times in the Civil War. (The closest there is is Franklin, which sees heavy casualties on the part of the attackers, and that was forts that had mostly been thrown up in a few days; the Richmond fortifications had been going up for months.)
Of course, at Williamsburg McClellan was originally not on the field because he was coordinating both halves of his army (the half marching up the Peninsula by land and the half moving up by water). First Hooker and then Kearny launched their troops into entrenchments at Williamsburg and got repulsed, and the decisive movement of the engagement was by Hancock (who captured empty enemy defences and turned the position), though when McClellan arrived on the field he had Hancock reinforced. McClellan says that Hooker did not do well, Hooker says McClellan did not do well, and Kearny says that neither of them did well; I think that to argue McClellan was wrong to say that the two did poorly you'd need to point to what they actually achieved beyond lots of casualties (chiefly their own, after all).
I should point that out - when Williamsburg kicked off McClellan was shipping half his army to Eltham's Landing to turn the Williamsburg line, speeding the advance up the Peninsula and preventing the Confederates from blocking them. That's a genuine operational move to accelerate the advance and avoid enemy blocking positions; one could call the movement up from Yorktown to Bottom's Bridge (13 days) slow, and perhaps it was, but we then need to set expectations for a large force in heavy rain advancing to contact.
I do sort of wonder how fast the Confederate units from Williamsburg moved. I'd expect them to move faster than the Union as they're moving into friendly space rather than advancing into enemy space, but it'd be interesting to know the timescale.