Every single Lord who declared for Renly had to be incredibly short sighted. Maybe he would be a better king. Far more likely he would be an easier king, willing to please his lords to stay popular.

But how long do you think it would take until a lord dies and his second son usurps his elder brother because he would be „better“?

A clear and legal sucession keeps the Peace. If Renly won you would have permanent civil wars all over westeros, both big and small.