Ex-SWE here. A metric fuckton of engineering in cloud goes into protecting systems against douchebags (e.g. rate limits, isolation). Those systems are not required for single tenancy systems in small orgs. Furthermore, those protection systems are often a cause of downtime. I think there are grains of truth in OPs suggestion you can roll your own on at small scale cheaper and with similar reliability. The key saving is it would never survive a multi tenancy situation.