If SMRs are cheap enough to act as backup to wind and solar, they are cheap enough to displace wind and solar entirely. And the contrapositive as well: if SMRs are not cheap enough to displace solar and wind, they aren't cheap enough to act as backup. The scenario where it's just a backup never arises in cost minimized solutions.

> If SMRs are cheap enough to act as backup to wind and solar, they are cheap enough to displace wind and solar entirely.

That doesn't follow necessarily. Wind & solar being the most cost effective doesn't mean you remove all backups just because they aren't as cost effective.

Its the other way around. If you have sufficient nuclear to act as a backup, then you have sufficient that you do not need the wind and solar in addition.

That’s the theory, it has yet to be proven in practice.

Even by their own claims, the caped may be smaller but the $/MWh is substantially higher than large plants, and will stay so even after multiple doubling a of production.