I'm not a teacher, but I think often when they talk about old textbooks, the issue is that they are physically falling apart, not just that they were published a long time ago. These books are in continuous use by children, so you can't compare it to a book you've had on your shelf for 40 years. As for lesson plans, most teachers don't have the luxury to just pick a plan they think is best. Every state, district, individual school might have its own rules about what can and must be taught, and teachers often aren't given much say in that.