I own a 2014 GMC Acadia with about 87k miles.

I own a 2014 GMC Acadia with about 87k miles.
1. Made an appointment for Saturday morning - we had trouble with our second car and were running late. When I called to let the service dept. know, I was informed a tech would not be there until Monday to look at my car. Why would the service center let me make an appointment when someone wouldn't even be there?
2. I brought my vehicle in Monday. They came back saying I needed a complete new transmission or to have my current transmission overhauled. Quoted me $6K for this. I had brought my vehicle in for it's regular maintenance over the last 7 years and most recently 3 weeks ago for an oil change. Why was my transmission issue just now being addressed? Why was any transmission maintenance not brought up previously? I feel as if my transmission maintenance should be something that is checked each time I bring my vehicle in; otherwise it's a very costly fix. I was told this is the owner's responsibility.
3. We consulted another GMC dealership service center; they said if the vehicle was still running, I would not need a new transmission, but definitely maintenance. This could cost up to $3K (significantly less than what Wiesner quoted). Why such a vast difference? My fear would be if what the second GMC service center said was true, but I chose to have my work done at Wiesner, I'd still be charged the initial quote of $6K.

This was just a bad experience all around. I've trusted Wiesner with my maintenance for 7 years with this vehicle and the last 2 years the service has plummeted. Before I owned my Acadia I had a Terrain, which I also only took to Wiesner. I've been told I need new filters, wipers, even tires (after the tires I had were only 6 months old!) when bringing my car in for oil changes, but never once was I offered any transmission service. We won't be bringing my Acadia back to Wiesner and will find another dealership to work with.