2) Being restricted to pure functions is not actually much of a restriction at all. Any impure input can be trivially replaced with parameterization over values

2) Being restricted to pure functions is not actually much of a restriction at all. Any impure input can be trivially replaced with parameterization over values representing that input and any impure output can be trivially replaced by including state updates or the updated state in the return value. Then your entire codebase can be entirely pure and you can let the language runtime or the compiler deal with that pure data at the very top level of the program, performing the IO as needed. This may be slightly more inconvenient than allowing impurity, but not enough that it would be by any means infeasible, and languages like Haskell have facilities which narrow the gap even further, so it's not much of a restriction at all.