You can't assume that the 3rd best QB this year is worse than the 6th best QB next year. If both would be worthy of pick 26, then likley they are similar players.
However, the guy you take this year will have a year in the system, learning from Stafford & McVay for a year, which would likley make that QB a lot better in 26'than the 5th or 6th best rookie QB taken next year. That is important since that player may have to play next year.
Agreed no assumptions on future classes. But we do know that Dart is rated 6.17 by NFL.com which would have been #6 last year. It would have been after Levis in 2023, after Pickett in 2022, after all 1st round QBs in 2021, etc.
Most of the time QBs that teams believe are franchise-caliber players don't make it to 26 because if a guy has long-term starting potential, a QB-needy team will take him early.
I disagree with your logic that a stronger rookie QB would be inferior to a lesser QB that has developed for a year because there are very few successful cases supporting the concept of a guy sitting, learning, and developing into a starter. We have Jordan Love and very little besides him in the last 15 years.
When looking at QBs drafted from 11-32 and outside the 1st round over the last 15 years, every long-term starter was ready to go sometime in year 1, typically early, besides Love (who BTW was rated a lot higher than Dart is) and Kirk Cousins. There are not many success stories of lower rated QBs that developed into long-term starters, but there are many dozens who didn't develop after sitting behind the starter.