Since making the scalar read-only doesn't prevent transformation (only assignment), there is no backwards compatibility issue relating to transformation.
Hence it's far more likely that legacy code will break if something like $_++ became illegal for an aliased input like 3+4 .
And that's exactly what the change is suppose to prevent. What's the point of making the change if it doesn't prevent exactly what it's suppose to be preventing!
If you say there's no point in preventing $_ = ... when $_ is an unassigned string, then there's no point in preventing it when it's an unassigned reference.