Equity compensation and what it's valued at is a big factor in choosing to work at Stripe. If they start letting RSUs expire, it will make ~everyone value their equity significantly less; Stripe would likely have significant difficulties finding and retaining talent.

It's not in the company's interest to piss off employees who can't afford to exercise their options without being able to sell them.

My company recently got bought by a privately owned company. Because of the ownership structure of the purchasing company we weren't allowed to exchange our options or let new ones vest so my company's board approved cancelling and reissuing everyone's options with an acceleration clause so they'd completely vest at completion so we'd all get a full payout. They did this because they knew if they didn't that there would be an exodus of employees before the purchase went through.

Employees represent a large % of the value of the company, if they all leave you just have a bunch of tech that no one else knows how to maintain or use. Institutional investors typically have such a large proportion of the shares that it's worth it to them to not just screw over employees because the employee sticking around and keeping shares is worth more than they'd save.

In some ways, yes, but I think if you game it out, you see how the company loses in a number of ways. First, repetitional risk. Second, it’s likely that a liquidity provider would emerge who could offer, basically, to buy call options from individual holders for enough premium to cover the taxes due on exercise. (Stripe would not want an unaffiliated 3rd party accumulating a large position)

If they don't let them exercise, the existing employees will rightly see it as funny money and not actual compensation. They won't be employees for much longer after that.

Especially since Stripe has steadfastly refused to go public so far...

Employees tend to leave when a major portion of their compensation disappears.

It would be terrible for the company.

Every single competent person would leave immediately and no employee would ever trust them again.