The thing is, exercise becomes a thing time is even a concern for only if it has been neglected for long enough. It is as important as bathing or brushing and the primary issue is that its not framed as equally important as those things. If it was, people would do it every single day. If they did it every single day, 15 minutes a day is more than enough.

However, given that people "graduate" their 20s without having done this, then everything you say becomes true, people end up so weak that they have to rebuild even basic core strength and posture. So practically what I suggested is useless except for maybe the next generation.