The magic of multi-tenancy is at the end of the day all about scale and complexity reduction. By eliminating so many variables associated with individual single tenant deployments - (you are eliminating the myriad combinations of hardware stacks, OS and infrastructure stacks, security, operations policies and people, etc) - you get vastly reduced complexity, improved cost effectiveness, and operational excellence for both the vendor and the client.
The key insight is you are literally taking N^N complexity of running software systems and reducing it down to a handful of variables by simplifying and standardizing everything to do with building, running and operating software. As always, this simplification, just like an industrial revolution, means that the output is more reliable, more secure and more cost effective - better in every way.
I remember at Oracle in the 1990's when we had something like 100 different versions of the Oracle database available for the various operating systems - our porting layer became as much a core competency as the database code itself - and we had literally tens of thousands of different versions and ports of the product out in the market. Now add thousands of different versions of Siebel, PeopleSoft, SAP and middleware running on top, with infinite customizations running on an unknowable combination of hardware it an unmanageable variety of data centers and all being run by IT people with literally no standards or commonality or even consistent skills and training in what they do.
It's almost remarkable anything worked at all.
At the end of the day, I think a good way to frame the single tenancy vs. multi-tenancy debate is to consider the differences between cottage manufacturing and industrial production. Clients benefit greatly from consistency, quality, scale and operational excellence.
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