1)This is why you’re usually in a bloc or protected by a high tier alliance (e.g Rose, Grumpy, tCW ..). Everyone in the top 50 is, except Pantheon.
2)As Ronny said, this is not worth it to decom and build up again, even for raiders.
3)Your scenario only works with 3 nations with 30 cities and low infra. Except Mythic, no one could do this.
Conclusion : Your scenario only works if you have a war between Mythic and Pantheon, and would still be as costly for Mythic as Pantheon.