I fail to see how subscription vs one time purchase costs are any way related to how much a customer costs to support, both situations have edge cases of demand

I fail to see how subscription vs one time purchase costs are any way related to how much a customer costs to support, both situations have edge cases of demanding customers eating up support staff time endlessly. If anything per-customer support costs go way up in a SaaS situation because of the (mostly artifical) introduction of having to run the software on your own servers.