The whole original point of what underpins FIDO2 was device locked, unphishable credentials. Wanting to export and move passkeys between devices is kind of counter to that. And I would argue vendors completing the attestation process are much more trustworthy than storing your own keys god knows where.
Services, or even microservices, are more of a strategy to allow teams to scale than services or products to scale. I think thats one of the biggest misconceptions for engineers. On the other end you have the monorepo crew, who are doing it for the same reasons.
On your note about resiliency and scale - its always a waste of money until shit hits the fan. Then you really pay for it.
Large tables take hours, if not days. I attempted a test case on AWS using souped up io2 disks (the fastest most expensive disks they have) and a beast of a DB server (r5.12xl I think) and it became abundantly clear that at certain scale you won't be doing any kind of in-place table updates like that on the system. Especially if your allowed downtime is one hour maintenance window per week...
Rails migrations really fail to be viable at scale. And its not really because of the migrations in Rails, its because changes in PostgreSQL get very very expensive. Things have gotten better in PG 15, but its still not quite there yet.
I've ran into an integer PK being almost exhausted in values twice now. In one of these cases I used pg-osc to rewrite a 8TB table on disk without downtime over a period of a week to fix the issue. In the other case it was about 6TB within 2 days. Its doable, but the amount of planning and time it takes is non-trivial.
I so far have no found any other viable solutions either. I keep hearing about logical replication but I haven't seen a great real-world solution at scale so far.
Self hosting likely makes it much much worse. I'm willing to bet a lot on the fact that most authors of these comments do not have any kind of experience with serious security or compliance programs.