1) The experience. 2) The job.
Having 10 years hacking on one system probably won't make you anymore productive than 3. At that point you're a factory worker, and you can't only put so much expertise into flipping a few switches and following traces.
However, having 20 years of diverse technical experience will make you more productive than 15 if the job requires the breadth you've developed.
I'm 12 years in and have moved into Startups for just that reason. You get to learn more numerous things, and you get to use all of them. I've been at my current job 6 months and I've worked with 4 languages, more libraries than I can count, 2 third parties, blah blah blah. If I went back in time even 3 years of experience, I would definitely be less effective. If I went back 5, I'd be struggling.