I think the reasoning behind your thinking is also flawed. You opined that Tunsil would likely be an upgrade over Smith because Smith wasn't all that good his rookie year.
Simply put, he would be a better option because he would most likely step in and play better immediately than what we say from Smith last year(which isn't that high of a bar), and because his ceiling is supposedly higher. It isn't about Smith being average to below average his rookie year. It's about how both of their career's project going forward. There will be a fundamental disagreement between us here, because we are on opposite ends regarding the prospects. I like Tunsil as a prospect and you don't seem to think he is as good as advertised. You like Smith and see him potentially being the guy at LT for the time being, and I have little confidence in his ability. That's fine, but my stance isn't to just replace Smith because he wasn't Joe Thomas in year one. It's that if you see a guy who is a significant upgrade not only in the immediate, but also the long term, then you don't hesitate replacing him.
Based on the fact that the vast majority of these players play poorly as rookies the chances of Tunsil coming in and playing great are very slim. Also, DSmith may not have played great last year overall but he did have some very good games and he showed flashes of dominant play. Plug in the Houston game from last year. He was pretty good in several games. As far as Tunsil goes if you think he is going to come in and play great as a rookie I believe you are going to be disappointed. He is a good prospect but I believe he will struggle as a rookie just like the other 99.9% of rookie LT's struggle.
To further hit on the first part of this response, it's not about Tunsil coming in and being great on his first snap. That wouldn't be my expectation, nor should it be anyone else's. To me though, Tunsil does represent a significant enough upgrade to the position, that at least warrants consideration. I don't see him being great in year one, but I do see him having a better career in the immediate and the long term compared to Smith.