> A clear majority of the UB problems with C could be fixed if the standards committee slowly moved all UB into IB

There is no such thing as getting rid of "all UB."

What behavior is the implementation supposed to prescribe for a write to an unpredictable garbage address you read from the network? It could overwrite your code. It could overwrite any value anywhere. It could overlap with anything. Prescribing defined behavior for absolutely everything would require defining a precise, unoptimizable 1-to-1 mapping to assembly code and disallowing any multithreading.