In your suggestion you fail to consider what prompted the 140 char restriction. The source of that restriction was that tweeter wanted the main source of tweets to be mobiles, using SMS. Now we can argue that they can allow people posting from the web to add some metadata but that would result in the inconsistency between the web and mobile protocols.
SMS has 140 bytes but not chars restrictions. When you use non-english letters you could not send 140 chars in one SMS. But you can use any 140 chars in Twitter.
So one to one mapping to SMS is not possible now.
When I enter SMS bigger then 140 bytes it's splitted transperently and the only consequence is increased payment for that SMS.
I think that nowdays the 140 char restriction is for keeping Twitter clean and readable, making it harder on the writer and easier on the reader - links are not hard to read and therefore there is no reason to restrict their length.
Also, users who actually use Twitter via SMS would probably not be capable of viewing the link anyhow so it could just say "(+lnk)" or something.
I used to manually type bit.ly links in from a tweet via SMS all the time. I didn't have a smartphone until a few weeks ago, and it was easier than opening twitter, typing in my password, finding the person's tweet, and clicking on it.
Shortened links are annoying and misleading, I've been wasting too much time clicking links I've seen already and links to sites I just don't like.