Given the injury history of all of the starters, plus the current usage of them, coupled with the fact that even before the injury bug really bit the rotation, they were already scrambling at times to fill a starting slot. That's not going to be fixed in one offseason. Realistically anyway. And as the last two seasons should have taught all of us, you can NEVER have too much depth.
And even if everything played out perfectly and you can't get everyone enough innings, that allows you to trade from that depth to acquire something else of need.
There simply isn't a thing called "too many starting pitchers".