Is there reason why retaining radiators is preferable to an air heat pump (either ducted air or mini-split)?

I guess it's cheaper to not have to install air ducting or indoor units if you can reuse existing pipes & radiators. But if you need to replace the radiators anyway, the cost might not be that much different? You'd get more precise temperature control, and cooling in the summer.

The roof cavity is not large enough for hidden ducting and the ceilings in older houses low enough that exposed ducting would be a head impact hazard. So you'd end up needing one installation per room. Expensive, especially when the expertise isn't there, and mostly used by commercial users.

Most old houses are not ducted and retrofiting them is very expensive, capital repair.

Mini-splits are noisy and have major problems getting heat distributed over a whole house.