"When Abraham (as) was tried by his Lord and he succeeded then he was made an Imam of the people",
Now I am sure that you know what Abraham (as) was before he was put through a test, when he succeeded he was made an Imam. This means he wasn't one before. And he was only made an Imam after being tried and after succeeding.
Now ask yourself this with a wide mind and an open heart, was he promoted??? Was he demoted??? Or was he given a title/grade/status of a similar level/stage?![Huh ???](https://forum.twelvershia.net/Smileys/default/huh.gif)
Technically, the answer to this doesn't matter for the question of superiority of an imam over a prophet. If he was demoted, it doesn't help you.
If we assume that there is such a station like imamate/imam (this is contested) and if we assume this ayah indicates a promotion ( also contested), then still it's not related to that question.
We might conclude that someone who is both a prophet and an imam has a higher status than someone who is just a prophet. We don't know anything about just an imam, which the thread was about.
Similarly when prophet Suleiman was made a king, we might conclude that someone who is both a prophet and a king has a higher status, we can't say that a king has a higher status than a prophet.
You can strengthen your opinion by showing that prophet Ibrahim stopped being a prophet when he became an imam.
As for what's the wisdom behind Imamate, I say what's the point of ending Prophet-hood if they are to be succeeded by "Imams" who are exactly like them heck even better!?
The usual response is that this is irrelevant as well, the wisdom behind it might be subtle, like why is there a gap between prophets and people are misguided?
A Prophets duty and job is to explain and clarify the measage, where as the Imams duty and job is to give further explanation and clarification.
Can you explain and clarify (further) what the difference is between explaining and clarifying and explaining and clarifying further?
As for the rest of the topic, the question of why something is as it is, is a hard one and you will get the same answer. To be fair, technically they have a point, not all reasons can be known. Positions should be rejected because of lack of proper evidence, not because of a lack of a valid reason
You can even question the entire idea of having a divine book as it serves no real purpose if you have continuously a divinely guided imam to refer to. A book can store information much longer than the lifetime of the messenger of that book which can be used to refer to as evidence at a later time. If you always have a guiding imam just ask him who can answer much more directly than a book.