Does only accepting PayPal for our SaaS make us look bad?
Does only accepting PayPal for our forms backend SaaS https://fabform.io make us look bad or out of date ?
Should we switch to stripe or a more modern payment provider -- will it improve sales?
The payment provider doesn't really matter, do whatever is most convenient for you to ensure faster release.
No, Paypal as payment provider is not a problem. Everyone knows it and it's widely accepted.
Fancy-ness is not need for something very technical as a payment is.
The design looks a bit flat, and the copy could be more clear. But I don't think the PayPal widget is a problem.