I'm sure the education at a good state school would absolutely be good enough for them. But I'm guessing you've never been past the average state school have you? We have 3 excellent state high schools in our area. 2 are rated Outstanding and appear on the Sunday Times Best Schools list. From an education perspective, I am confident that the royal children would do just fine at either of those schools.
But the practicalities!? Oh my god.
Both schools are MASSIVE (6-8 form entry depending on year group) and while well funded and looked after, the additional costs to create the right kind of security - doors, windows, fences, walls etc would be massive. Both schools are in residential areas where traffic and parking is already a nightmare - an additional massive amount of traffic due to rubber neckers, additional security, media etc would be horrendous. I imagine that their security worry also about things like line of sight - so what houses are nearby/gardens are close to the school etc - in densely urban areas, that's going to be a lot more complicated to manage (I don't think it's a coincidence that as the children have started getting older they've moved to a more rural setting for their private school). The schools are both hugely diverse which I would say is actually a good thing, even more so for the royal children. BUT... it does make being able to figure out how to monitor risks or spot those children/families who might target them a lot harder.
Even extra curricular - both schools have strong extracurricular offerings. But NOTHING like what's on offer at a private school. This is more of an issue becuase again, if I was the W&C, I would want as much of what my children do to happen at school where security is already in place and to avoid too much driving around, background checking on groups etc etc etc.