In reality it often means these systems are built in bad, ineffective way. Most likely in bad, ineffective languages/technologies. You don't need 17 years to be able to contribute to Apache Spark. You would not call that a small project, would you?

No, you can stick around.

I wouldn't consider someone who spent 5 years on AWS, especially if they moved internally, out of date.

Whereas I've done nothing remotely close to where I want to be in about 3 years. I feel very scared about my skills, especially since I'm not even 10 years into my career.

I spent 9 years at my first FT job out of college at a hedge fund known at the time for culling the bottom 10% of performers annually at the time. That long of tenure at a place with that reputation has openned more doors than it has closed, especially within the finance industry. Lately, I've been getting 4-6 emails from local recruiters per week.

An aside, even when you spend that long in a single position, you do need to keep learning on new and relevant technology; I attended every on house training seminar I could, which helped a lot, and spent a good portion every day reading. I spent 7 years at that company in the same effective role, and never changed job titles, but saw a >200% increase in take home pay in that same period.