I don't see much talent being pulled out of Shute Shield. But it's not what gets picked, it is whether someone with an understanding of who has a possibility of stepping up. I think we pull players from schools and those who are put through some wider squads ie under 18s or colts. There are many players who are average at school but become very good later on. I don't think any of these guys over 25 get a second look, particularly props.
I did a fair bit of subbies rugby as a ref, and some guys were running around in those comps that could have stepped up, but nobody ever approached them. I remember a 10 playing for a southern suburbs subbies team, he was as good as any 10 I played with in my time in senior rugby. I asked him if he ever thought about stepping up, he said no club or person, apart from me, had suggested it.
I'm no selector, never had experience in it - so take all of this with a grain of salt.
Without applying this as an absolute blanket rule because of course there are exceptions, I feel that
most players generally show whether they are made from the right stuff pretty early on, hence why we see the majority picked from schools and put through the pathways you've alluded to.
Without doubt a lot of players mature and get better with age but I would strongly doubt that they'd get so much better that they could make the step up. I know talent ID is labour intensive, so professional teams probably feel they are more likely to get a ROI by looking at younger talent, rather than club rugby. I know there are examples of older players getting a crack, but I think that would be the case in a larger, general sense.
No idea what your own eye for talent is like, but in the case of your subbies 10 example - I'm going to guess the answer to why he never got further opportunity is probably the obvious one; he probably wasn't actually good enough to.