Some reasons to go to cloud go up in smoke
You're right of course.
But, "people are building equivalently robust systems using cloud parts all the time" is rare. Yes, people build, people shift, and it's mighty convenient -- but people downgrade.
Many large customers try shift their on-prem data center to the cloud looking for savings in these areas:
1 - data center infrastructure (airconditioning, fire-suppression, UPS, backup generation)
2 - manpower for infrastructure management
3 - money in general (because the cloud supposedly is more efficient)
But for (1), the cloud provider has near-equivalent expenses; (2) was never that high anyway; and as for (3) the cloud provider wants those juicy profits all for themselves!
So once the honeymoon is over, the corner-cutting begins:
- "Do you need premium SSD? What not standard SDD?
- "Why use SSD at all for non-prod servers?
- "Why so many non-prod servers? So much disk? So much memory?
- "Can you switch DEV/TEST off at night and boot in the morning?
- "Multi-zone redundancy?? But the cloud is so reliable!
Then come the cloud VMs crashing without notice. And 'live migration', an Azure specialty -- a VM freezes as it's migrated to different hardware. Too bad if you had clustering software with a heartbeat detector sensitive to timing delays.