Aren't the batteries and electric motors driving the grid fins at the top of the booster? That and the entire interstage are gonna get blasted with the thrust plume of three Raptors. Reinforcing them enough that it doesn't affect planned reusability targets could take a bigger bite out of the payload than they get from hot staging.

That said, assuming the booster doesn't get royally annihilated immediately, they'll surely do a thorough analysis on just how much damage the booster takes. Might be that hot staging doesn't work out for regular use, but they'll keep it on hand for launches that need every last bit of delta-V.

I think Soyuz boosters currently do hot staging, the interstage is open IIRC.

You are correct. I believe most Russian rockets have used hot staging. It may be destructive, but it works.