What's not discussed is "the increased savings from moving to an open source knockoff of 37 signals' offerings", given that you're not just paying once, but every month.
In extreme cases, businesses with cash flow problems are just like poor people, who are forced to patronize ripoffs like check-cashing places, payday loan sharks, and Rent-A-Centers because they can't afford the minimum balance or down payment on a better solution. They live day-to-day. So do businesses in trouble.
From the seller perspective it is normally easier to get folks to pay a smaller amount for one month's use than an annual or perpetual license fee. You can always offer annual and multi-year licenses with a discount structure in addition.