Yes but the thing is this is one of the few crimes which the automatic and necessarily punishment is spelt out in the Laws of Chess. If the arbiter is to follow the Laws of Chess (which they must according to the Laws) then they must rule the game forfeit by the player whose phone rings. (That doesn't necessarily mean the other player scores a win.) To do otherwise is to now follow the clear intention of the Laws.

With regards disproportion, I see it like this. The rule makes made the punishment that punitive so as to ensure players took every measure to make sure that their phone doesn't ring. Some people forget and get caught out, but that's life. Without the rule, mobile phone interruptions would be much more common.