The alternative is someone having a meal that's been reheated or kept warm is that what those of you who only cook one meal do?
Occasionally.
I'll do that with something like a chilli or a casserole but lots of meals aren't nice reheated.
I agree, and, particularly during the Winter, we tend to have meals that are easily reheated - chilli / curry / bolognaise / casserole / cottage pie etc, and then those that aren't so good at being reheated, we'll allocate to a day when people are there, or aren't eating with us for some reason, or they might say, 'don't cook for me today, I'll grab something when I get in'. It isn't rigid, for 100% of our meals, but it is the default setting.
I also think dc need to learn to cook a meal before they leave home.
I totally agree with this.
Mine have been cooking for the family, on a rota basis, each week from when eldest was 14 (youngest was 9 and obviously got a lot of support at that stage, but in her mind was still part of the rota). One bonus of this, that I hadn't anticipated, was that it cut out a lot of the moans about food that was put before them. They knew it wasn't always easy and that things sometimes didn't come out as nice as it might, and became a lot more understanding of that and a lot more accepting of everything not being cordon bleu.
It's a very different scenario when you have teenagers with their own lives
Not really. I've raised 3 to adulthood. All had commitments (sports, music, drama, Scouts, Volunteering). Youngest is 20 now. All also worked shifts during 6th form. As WalkingonSonshine say, , just like my siblings and I did growing up, my dc have learned that , no, you don't always get your 'favourite', but over the week you will. When it isn't your turn, you just get on with it, or offer help to substitute something in a meal.