The can't afford it exception is disappearing soon, as it isn't true for any business. Total setup costs for STIR/SHAKEN are under $2000 these days. Providers that lack control over the network infrastructure (i.e. they don't have the ability to control the stir/shaken headers so by definition they can't spoof numbers) will likely continue to be a thing as changing it would force pretty much every small business in the VOIP industry out of business and allow only large companies to be VOIP service providers.