Sorry to bring the Borders route back to this. However, we were warned that the bus pass holders would not desert the "free" bus for the "pay" train. In practice many did (pre Covid at least) with the vast majority of pensioners travelling holding senior railcards (implying that they have "bought" into the railway).
Again, anecdotally at least, on the Borders trains that I worked pre Covid, appropriately ⅓ of all passengers had tickets to/from destinations beyond Edinburgh. I would assume that Caernarfon would draw a proportion of long distance travellers too.
But ignoring the fact that before Borders the journey to the nearest railhead was 90 mins minimum by bus. With Caernarfon it's 30 minutes. That's why the two aren't comparable.
Caernarfon's travellers will already be using Bangor, because it really isn't far.
In fact I've known people who live in Criccieth to drive to Bangor when heading to London rather than use the Cambrian Coast line.