That doctor fed you a load of malarky! Correction; he tried to feed you a load of malarky, but you were too smart to eat any of it! Every knee is different, and every knee recovers at a different rate, therefore your knee will recover at it's own pace, and not at anyone else's pace. Some people have their swelling dissipate at a relatively quick rate so have more flexion earlier than someone else. Some knees take longer to heal and thus continue with the swelling longer, hindering bending progress to some degree. There is no hard and fast rule on how long any given knee should take to bend to any specific degree. Granted, if your knee doesn't bend enough to live your life by a year after the surgery, then maybe there's a problem, but you're at just over a month! Your knee is still in shock over what happened to it and hasn't even had a chance to figure out how to process the assault that is a TKR. This recovery takes, on average, 52 weeks, and you're only at 6 weeks. By around 3 months most people begin to feel much better, but the healing is still going on inside your knee.
Here's my silly attempt at an anology...we had a new roof put on our house a couple of weeks ago. The first 2 days there were 7 men on my roof removing the THREE layers of our old roof, replacing any wood that needed replacing, and so on. The next day and a half there were only 2 men on the roof at any given time, nailing on the paper, finishing up the wood replacement, putting on the shingles, and so on. The first two days were the hardest work, so there were many more men on the job, the last day and a half was still busy, but only needed two men to complete the job. By the same token, my next door neighbor, who has the same sized house, had a roof done just a couple of months before mine, but her roof took over a week to complete, as she had much more damage up there and needed a lot more wood replaced, even though she only had one layer of old roof that needed removing. We also used different roofing companies, so their methods were likely a bit different. But, in the end, we both have beautiful new roofs that will not leak for years and years to come! And that's the goal-to have a knee that functions well for years and years to come, so, if one knee takes longer to reach that point than another knee it doesn't matter-what matters is the end result.
At first, your knee has a ton of healing to do so a great deal of your body's resources are sent to that knee to begin the healing. As time goes on, the acute phase of healing is behind you, but you still have healing taking place, you just need less of your body's resources sent there to do the work. But it still sends some to your knee while you are still healing. At some point the job will be finished, and your knee will be done healing, but no one can tell you exactly when that will be, they can only give you estimates based on what was done, and how bad the damage was. After the surgery, your knee takes over and repairs the damage in it's own time.
Doctors sometimes say they'd "like" to see 90 degrees at 6 weeks, but any doctor who threatens a patient with an MUA or they'll have a semi-frozen knee for life needs to have a very large penalty flag thrown right at his feet! I don't know what gets into some of them, but whatever the reason, he is totally wrong. I don't care how good he may be as a surgeon, as someone giving recovery advice, he needs to move into the new century. While I agree you need a new doctor, I can't help but wonder what would happen if you saw him again and stood up to his bullying. I've always hated bullies, and a doctor, someone who took a vow to "first, do no harm", bullying his patient, is doing great harm and needs to be called out on his behavior!
Let your knee recover and help it all you can by icing, elevating and resting in between your daily activities, and it will reward you by doing what it needs to do to heal at it's own pace. I also suggest reading the activity progression article more than once to remind you not to do too much, or too little, at any given point in your recovery. Good luck and keep us posted!