Can you think of many examples of a SaaS provider who regularly keeps old versions of a product around for customers to use?
A far more common scenario is that new versions are rolled out to everyone, without offering a choice, as soon as they're considered stable.
Older versions consume resources and require staff to spend time on operating and supporting them. Those resources could be used to run a newer version.
The tl;dr is the simple economics of any SaaS product.
If you want to be able to run old versions indefinitely and control the resources assigned to it, you need to self-host (an open model).
> Can you think of many examples of a SaaS provider who regularly keeps old versions of a product around for customers to use?
Sure. Blender and Ubuntu offer long-lived old versions of their software that get regular fixes.
Neither Blender nor Ubuntu are SaaS. You're just confirming my point: if you want to run old versions of software, you need to host it yourself.