All voting systems must provide assurance that they work accurately and reliably as intended under federal (U.S. EAC) and state certification and testing requirements. Many states conduct extensive pre-election Logic & Accuracy testing on every machine for every election. Paper tapes and records also allow for auditing or review as a fail safe.
Removing machines does not guarantee security - it just changes the risk profile. There are plenty of examples of election fraud being committed using paper while there are zero documented instance of any voting system being hacked to steal an election.
It does not take a cyber expert to “hack” a paper mail ballot. All you need is pencil. It takes the proper procedures that must be followed and the hard working great election officials to insure safety and security for all systems of voting in every state.