Hmmm....I have not heard of the recommendation to keep them in the brooder 6-8 weeks. In fact, I've heard that chicks kept in a brooder that long end up less hardy than those that are acclimatized sooner. Personally I've never had a chick in a brooder past 4 weeks of age, and that includes some that I hatched in January several years ago. By the time they are 2 weeks old, I'm looking for a nice afternoon to give them some time outside. By 3 weeks they are outside almost all day, and sometime between the 3rd & 4th week, I look for a night that is forecast to be mild, and instead of carrying them back to the brooder at the end of the day, I carry them to the coop. From then on, they are outside in the grow-out pen by day, and in the coop at night. I've never lost one to cold, and in fact, have a pretty hardy bunch, all told. Right now I have chicks in a brooder in an unheated 3-season room, where they've been since hatch. We've had days below freezing but their only heat is an EcoGlow. Even when the measurable temperature in the 3-season room is in the 30's, the chicks spend very little time under the EcoGlow - which matches my observations of chicks raised by broody hens. I also have several hens raising chicks right now and despite our wintry weather, they spend an enormous amount of time out from under her, only running under occasionally to get warmed up.
I know that anyone who puts a heat lamp in their coop thinks they have it hooked up securely, yet, as I said earlier, coop fires happen every year. I heard of several this season alone and some of our fellow BYC'ers lost a lot of birds in those fires.
Another consideration regarding providing heat in the coop is that if they become dependant on the heat, and then you have a power outage, you are more likely to lose birds to cold, than if they hadn't had the heat on in the first place, and had been allowed to acclimatize and learn how to keep themselves warm.
I agree with just about everything you said. Mine have been going out to the 'grow-out' pen since about 2 1/2 weeks, depending on weather. Temps. here in Central AZ. have varied from 28 to 78. I know mine should be in the coop now, but it's really me that's not ready! Coop fire is the second reason. There are nights when they would need extra warmth. For instance, yesterday was hot! Today, there is a wicked cold wind. Mine are 7 wks. plus and still in big cages at night in the house. I keep it very cool for them and I bundle up. Shame on me! But man, when they are top of me in the rocking chair watching movies and fighting for their favorite spots, I'm happy as a clam. I'll be lonely when they go out!
I love that term 'grow-out' pen! I've been calling mine the redneck playpen!