I'd recommend watching some of his college highlights. I watch a lot of both Big10 and SEC football, and any team that routinely plays ND. Unfortunately or fortunately lol I have several buds that huge Auburn fans, so I've both watch a lot of games, and heard a lot.
Not sure how you can say he wouldn't make a better OG, if you've never seen him play OG.
Here's NFL.com's combine rating on him.
Making it work through athleticism is an incorrect statement and does a huge disservice to Smith. Smith is making it work at tackle by working hard and becoming a tackle. I'm not sure what makes a "true tackle" but to me a guy that plays tackle and plays it very well is as much of a true tackle as anyone else.
As far as AC (or AT if you prefer ) aging... yup, every player on the team is aging so that is not a shock. But he is still just 30 and tackles often play into their mid to late 30s so that is not much of a concern. Not to say they won't draft a tackle in the next couple of drafts, but I doubt they will try to find a starting tackle in the next couple of drafts.
I didn't watch him much in college, I typically watch Big 10 football but I have watched him closely in the NFL and if the "experts" think he would make a better guard then yes I disagree with them. While he would be a good guard in a zone blocking scheme like the Colts use, his lack of lower body strength and quick hand punch off the snap would hamper him going against NFL DTs. Additionally, Smith does better when he has a bit of space to work with before engaging and that would not happen often at RG.
Again what "most" people on this board or in general think has no relevance on anything. It's not unheard of for a tackle to move spots but it is uncommon, it's happening a bit more in the past few years as the skillsets for RT and LT are becoming closer because it's not just the LT that is dealing with the speed rushers, but it's still not preferred among oline coaches and teams. that is why guys like Haeg are considered valuable assets to a team, he's not a great lineman but he is good and he can switch sides seamlessly and that is rare.