Our government is structured in such a way that elections are run at the local level, which can be seen as a strength. Imagine how dangerous it could be if one company manufactured every single voting machine used in every single polling place.
How many companies does it take to cover 90% of the electronic vote? Five companies are more difficult, but hardly impossible to manipulate. One company may be easier to audit, on the other hand.
In some ways, one company would be more difficult to audit than multiple companies, as the auditor wouldn't have any frame of reference.
If we are looking at a future with pervasive electronic voting machines from multiple manufacturers, I would insist on having multiple different kinds of machines in each polling place. That way, any meaningful differences between machines would stand out.
