I buy the rationale that saltpeter no longer came from natural sources, but could be made synthetically.
As for rubber and infantry: rubber soled boots, rubber overshoes, rubber blankets, and rubber used in butts and recoil mechanisms in rifles all made infantry much more field-worthy for trench wars in any weather. It doesn't quite revolutionize infantry tactics, but it certainly modernizes the equipment. I think it is reflected in the game that infantry are a bit tougher than riflemen, but their tactical use doesn't really change except that they may go on the offensive in the field more often.
(p.s. long time reader, first time poster!)