Second best option is ability to easily create prod environments, and then give those to your biggest customers (bigname.figma.com) etc. No single figma customer will go beyond an i3.metal for the DB, or the app.
They also stated it was too risky to migrate data stores on the timeline they were working within
Those all seem like measured engineering decisions AFAICT
Do data dump from prod for initial sync and then setup replication from RDS to new cluster. Once synced do switch. Then you're off RDS and can shard on Citus.
The coordination alone usually takes months
This issue is so pervasive that we've all just assumed that it must be necessary.
I just read the article and from what I can tell the Figma team made a somewhat reasonable sounding decision