So, you think these businesses took an existential risk with their reputation for the ephemeral benefit of looking cool? You are seriously underestimating the acumen of the folks who each went from zero to critically central providers in a cutthroat industry in less than 10 years.
Stripe/Paypal's constituents include their employees, businesses, and end users, all of whom have lots of choices about which network to align with. Their job is to project both a service and identity that align those folks to maximize the value of their network effects. They decided that the wedge of people who are worried about deplatforming (either Gab specifically or in some holistic way) is smaller than the wedge of people who would rather not be in the same ecosystem as Gab. I seriously doubt you know those audiences better than them, but if you're right then you have a great idea for your own fast-growing business.
BTW, the above doesn't have to be interpreted some cynical, 5-dimensional chess game analyzing user segments. Individual and brand identities and ethics are part of how people make purchasing/participation decisions, whether they are calculated or sincere. Sometimes a part of what makes people and their companies succeed really includes values they articulate that don't seem like part of the core product. You may dismiss those ethics as chasing some sort of trivial coolness-or-PC-ness, but enough other people take them seriously to move a lot of the world's money and time around. If it's tempting to dismiss that chunk of peoples' motivation as some sort of NPC ignorance (edit: apologies for putting words in your mouth if not), you might actually be defending your lack of effort to understand and adapt.