So what about these folks who have spent decades by the loch and never have seen a sign of Nessie? Is this a proof of Nessie's non-existence?
Not at all.
Firstly, all the major places of habitation around Loch Ness such as Fort Augustus, Drumnadrochit, Foyers and Dores are not built on the shore line. In fact they can be quite a distance from the shoreline as in the case of Drumnadrochit. So you could spend your entire life in your town and never actually see the loch!
Secondly, it is surprising how much of the loch is not visible from the main roads surrounding Loch Ness. In the case of the south side of the loch, hardly any of the loch is visible and the north side is not great either. In fact, I would say that as you drive around on the A82 and B roads, not even 10% of the roads present a decent view of the loch.
So in the case of our local residents driving off to their jobs in the morning and coming back at a speed of 50mph or more, any view of the loch will be short lived and of low quality as they have to keep one or both eyes on the road ahead.
So perhaps the typical inhabitant spends only minutes per day looking at Loch Ness. Over thirty years that would add up to 850 hours for about 5 minutes per day (allowing for being absent on holidays or being sick). And remember this consists of glances, focussed on other objects such as boats with no systematic scanning of the surface that serious Nessie hunters would employ.
In that light, I am not surprised the vast majority of Loch Ness natives have never seen anything!