Is the capacity of the network to handle large blocks 8x bigger, or 20x bigger? Is there a high degree of confidence that it can handle 1000x bigger blocks in 15 years?

I think the answer to all of these questions, as a purely factual technical matter is no.

As I said I would support a smaller increase and a smaller automatic rate of increase going forward. But an immediate 8x along with 2x every two years is reckless, just as an immediate 20x was (in fact worse imo)

The other thing nobody seems to address is that even these rates of scaling don't really do that much. 8x a few transactions per second is still only a few (maybe 20) transactions per second. Even 1000x does not reach globe-spanning capacity with everything everyone might want to do being handled on-chain. So all of the off-chain and demand-limiting fee increases stuff that big-block proponents are worried about is going to happen anyway.