The few who continues to grow after 10 years gets into extremely well paid positions and have no problems getting jobs. We aren't talking about them here, we are talking about the majority who never grow out of their senior engineer position. They aren't more valuable after 20 or 30 years than they were at 10 since they stopped growing, and this is the majority of people in every field.

You don't fix age discrimination by saying that old people are better, because in most cases they aren't, instead you fix it by saying that if you have 2 persons with similar skills then you shouldn't automatically pick the less experienced younger person. If you only hire old developers when they have all of the awesome skills people tout in these threads then you will mostly hire young developers since so few developers have those skills no matter what age they are.