The problem is the starter needs a lot of practice snaps to get ready for the 1st game.  Ideally in yesterday's scrimmage, the starter should have got at least 20 snaps, backup gets about 10 snaps.  And 3rd string QB gets 6 to 8 snaps. 
The coach has to worry about getting the starting QB ready for Game 1.  The hypothetical situation about injuries - you worry about it if and when that happens.  
Only way Siemian or Lynch gets better is if the coach puts the trust in one of them and lets that QB takes the majority of the snaps with the 1st team.