50% is already too large. The problem is that if that dominant player does something (like raise prices), you cannot escape them. The rest of the market does

50% is already too large. The problem is that if that dominant player does something (like raise prices), you cannot escape them. The rest of the market does not have the ability to absorb all the people who would like to flee the dominant player. The problem is Always Late(tm) inventory combined with process optimization. No competitor has significant inventory or excess capacity, so nobody can respond to sudden consumer changes. Covid toilet paper shortages were a great example: industrial toilet paper suppliers were unable to deal with the shift to consumer and consumer toilet paper suppliers were unable to deal with the demand shock. What the number should be is up to some interpretation, but I suspect that no market should ever allow a player with more than 25% market share no matter how you define "market".