I genuinely appreciate this as a Fastmail customer and JMAP enthusiast! Are SaaS companies more nimble in their ability to provide this type of support? Most certainly. I'm just uncertain it applies broadly to SaaS companies in general.
It's more likely that something like Cyrus or Perl will be a key part of the business at Fastmail than at some random similarly sized shop where email is something they use rather than a key part of the business, and hence worth spending time/money on.
And there's economies of scale. A few cents from the payment by each of our customers adds up to a decent amount of money to spend on an open source project.