Renault and Mercedes pushed for change. Ferrari and Cosworth didn't want change.
The teams wanting change prevailed and we lost one engine supplier.
One more engine supplier is supposed to come in next year getting us back to where we were last year.
Cosworth was never going to be financially sustainable with only one customer team. We can go around this as many times as you want, what the teams originally wanted was a 1.4 liter inline T. They (meaning all teams including Ferrari) agreed on a 1.6 liter T and a one year extension.
I bet I can come up with a much longer list of arguments for changing than the only argument for keeping them which is "they sound better".