Generally the idea is that historically there were frequent fires which kept the available fuel to a lower level.

Humans started fighting wildfires, and for decades we were pretty successful. That resulted in more fuel buildup then happened historically.

When those fire finally burn, they often burn hot enough to destroy trees that used to survive the fires.

I don’t know what the fuel levels were in this area specifically, but yes, it seems plausible that the brush might not have built up to today’s levels before.

All that is true, but we humans also tend to cause fires more frequently too? I live in a fire zone (I'm writing this from a hotel room that I've been forcibly evacuated to) and while Lightning gets the blame this time (I'm sure PG&E execs are quietly sighing relief), it doesn't seem to be the cause most times.

I wonder what the quantifiable overall effect is.

I think it is qualifiable if not quantifiable in that brush is building up. Perhaps more fires are starting, but the net effect would have to be less frequent burns of a given area for brush to build up there.

Modern Wildfires are not always extingushed outright and allowed to burn because the fireservices are acutely aware of this.

Very possibly not. Brush builds up because we suppress fires, the ecosystem is in a fundamentally different state than it has been for most of the last millennium.

Yes. Efforts to prevent and extinguish wildfires create the conditions for exceptionally severe wildfires. Invasive species, monoculture and climate change have all increased the potential severity of fires.