What would be the reason for them to boost this fee right now? Is it just a pure profit play? That is, they think they can extract more cash from the people who are reliable instant-payout users (rather than losing them to standard payouts)? Does this portend anything for the company, in the way that not backfilling positions means that layoffs may be imminent? Or perhaps a corporate transaction like an IPO?
For any payment processor part of the fees you pay go to offsetting fraud (it is a cost just like servers or people). Instant payout is much riskier because if a bad actor is using Stripe to cash out stolen credit cards they have less time for the banks to detect and report it before the money is gone. As a result it has a higher cost to the company.
The reason credit card fraud for charges costs money to processors is because of charge backs. I believe charge back fees originate from the card networks themselves (Visa, MasterCard, etc). These processors also enforce a variety of limits when it comes to chargebacks for each merchant. This means if you're the layer between the merchant and the network, the merchants generally will rely on you to pre-emptively detect fraud. Those systems all cost money too.
As far as I know when it comes to payout rails such as ACH, real time payments (RTP), Zelle, I don't believe the payment processor holds any liability for fraudulent transactions. In other words, if a fraudulent payout occurs through stripe via RTP then The Clearing House banks aren't going to come after stripe for the money. They'll tell the end user "whoops, should've taken better care of your digital info. Bye!"
source: Worked at a payment processor and worked on payout rails and integrating with banks. Also do work now as an end user of a different payment processor that does charging, payouts, etc.