I'll ask you to consider this: How many quality fifth starters are there in baseball? How many quality sixth starters do you think there will be? I see it as easier to find FOUR quality SPs than FIVE or SIX.
Yes finding four is easier to find. no argument there whatsoever. The issue is durability. Say we did a four man rotation. How long before arms started falling off that the disabled list was pages long? Then what pitcher in their right mind that is major league worthy would ever sign for this team?