I'm sorry to hear that.

You are supposed to service that transmission every 60k miles. It is a "sealed transmission" for the warrant time and after that, as you experienced it, whatever happens, you are on your own.

I have a question for you: 

How often do you change the engine oil?

To answer your question. No there is not a problem with Camry transmissions. The problem is with Toyota and ALL OTHER manufacturers claiming no transmission fluid change is necessary as it's "lifetime" trans fluid, sealed trans, etc...blah, blah, blah. BS ! Transmissions need to be serviced regularly changing out the fluid (and filter if there is one) just like all the other systems in vehicles. 

my transmission quit on me

Began slipping? Harsh shifting? Wouldn't shift into gear? Check engine light?

I’ve owned the last 4 Camry models. I do all my own maintenance. My Camry well kept.

It is NOT well kept if you have never replaced the transmission fluid.

The correct maintenance interval is 20k-40k miles transmission fluid and cleaning the tranny rad every 1k-5k miles.

I usually recommend 30k mile interval and checking your ATF temp using OBDII after long / hard drives.

What do you think is the biggest reason my transmission went out?

That's cause Toyota, "cares" more about the environment than you - the owner.

This transmission is built to have maximum efficiency and keep the RPM low, they didn't care to make it last.

So you see A LOT of design choices meant to increase efficiency that harm longevity.

Another aspect of why it's crappy, is cause the focus moved from the powertrain to the exterior and interior.

On a Peugeot with this exact transmission model, you get the 300NM capacity variant with 1.2L engines or under,

You get the full 400NM capacity variant with 1.5L and with 1.6L you get the tried and true 6 speed not an 8.

BUT, Toyota puts the worst and smallest version on 2.5L / 3.5L engines - and the results are as you'd expect.

Usually they fail because of bad maintenance, overheating, or bad driving habits.

Talking to people they say there are a lot of cases where it just looks like manufacturing defects.

Is there a transmission problem with 2019 Toyota Camry models?

It's not a "problem" it's just how new Toyotas are, somehow these are still the best new cars.

I just paid out $7k cash to put in a used transmission (...) What a rip-off!!! 

You did get ripped off, $7k for a USED unit? That's why you should never go to a dealer.

I wonder how much of that is labor? I'm unsure if that's applicable to the XV70 Camry but on some new Toyotas dropping the tranny requires dropping the subframe and that's many hours of labor. (just not enough space to separate the two)

Usually transmission replacement on cars FWD sedans that aren't built to torture mechanics is usually 3-4 hours.

If you didn't change the trans fluid at 60,000 per the maintenance schedule, you voided your warranty.  Simple as that.