The evidence that PIK-SNN wasn't profitable enough so Ryanair canned it? The evidence that GLA-SNN failed with EIR, as did EDI-SNN. The fact that Ryanair walked away almost entirely from SNN as they were forced to pay money to use the airport, and only now are growing again.
It's not about what you think "might work", it's about "is this short less than daily Scotland -Ireland sector the best use of my asset or will it be more profitable being utilised to the sun"? That's why the mix of routes at PIK was changed. What "evidence" do you have it will now "work"?
Btw does €20 return even include APD and how recent is that figure?