You mean to say that adapting these much larger signatures will cause a collapse in security unless we increase the block size, right?
No, I mean that we could not upgrade at all, and leave quantum computing be a threat. I don't see this happening, but it's also another option.
At first glance it seemed to me that you are trying to say that changing the block size changes the mining economics, which I don't agree with.
I hadn't thought of introducing a new field as stwenhao said, or changing the witness field we currently have. Maybe it's possible to increase the absolute block size without interfering with mining economics.
What's the solution then? Can't find. Maybe quantum fud wasn't all fud at all. Time to panic?
It might be a FUD that it will be a threat anytime soon. The problem is that, after many years, it might be too late until the whole network upgrades and endorses the protocol changes. It takes a hell of a lot of time to get consensus in bitcoin. We need to upgrade in the next years, but keep migration optional, if quantum computers do not prove to be threatening.