- Russia's S-400 air-defense system is feared.
- But Ukraine has been able to destroy some, including, it seems, with older weapons.
- Experts say it's still a formidable system, but the West could learn from Ukraine's wins.
The performance of the Russian military's top air-defense system in Ukraine has shown it's vulnerable to even some older Western missiles, and wins against it could give the West new ideas on how to defeat it, experts say.