Too bad for eBay. I think they'll regret this decision. Ultimately this will likely hurt eBay more than Paypal. It took a long time to build up peoples trust in Paypal.
I don't think that's going to be an issue as, unlike PayPal, Adyen is a 'back end'. So whereas with the current set-up, you're very obviously paying through a third party (i.e. PayPal), with Adyen you won't actually see any third party involvement, you'll just make/receive your payments through eBay itself - no taking you off to a third-party site, no third-party logos etc. No sign of any third party not to trust.
That's my understanding of this anyway.