I have a 50+ yo gas furnace and a 7 yo, fully functional A/C. The furnace has been repaired too many times and the parts are now hard to get. The blower is bad. We have decided to replace the furnace. As the bids came, all of a sudden, the estimates escalated as the majority of the bids included an A/C because "you have to replace both". The A/C works well and was just serviced with nothing wrong. Why do I need both?

You do not need both it is that simple. If you are going to get new A/C then I would suggest getting both the outside unit and the indoor coil replaced but if the A/C you have now works good then I see no reason to replace it.

There is no need to replace the A/C, It sounds like they just want to sell you a complete system.