As for in-system jumps, I believe the only reason we don't have them is the interdiction minigame. Which nobody uses anyway, because you'll either submit and jump out or submit and fight (so it can be removed whatsoever with target submitting becoming the default outcome). It's obsolete. Still, it's there and interdictions are possible, so FD decided to artificially provide time for them to happen. Which is a shame, because I'd rather jump between astronomical bodies (not only stars, but planets and stations - basically anything you have in the navigation list) and on top of that have a good reason to go into supercruise, thus providing opponents with interdiction opportunities. I'm not talking here about instant teleportation, rather a sequence (similar to current witch space) when others can actually interact with your ship. They just have to be quicker about it
Such solution would give FD the opportunity to re-think interdiction mechanics. Frame-shift disruptor guns perhaps? Where you hit your opponent and they are out to normal space like if they submitted with the current system. That's a subject for a different thread though.
My point here is that there isn't really any reason for supercruising to be the main mode of in-system travel. The only thing it does is consuming time, it doesn't provide players with any value worth that time. I'd rather jump between the points of interest (normal space -> in-system jump -> normal space) and use supercruise for discovering objects not showing after the honk (USS and such), getting to mining locations other than extraction sites and so on. Current in-system travel speaks volumes about developers' game design philosophy, which is the main reason majority of the threads on this forum exist - most suggestions and requests focus on either reducing the grind or providing better ways of filling the time spent in game.
Anyhow, in-system jumps change nothing when it comes to trading. As mentioned by someone before, you will typically trade between stations relatively close to the main star. My typical trading routes used to include stations within 100Ls from the entry point or not that much further away.
If you were able to slide / warp / hypercruise (or whatever you want to call the in-system jump) to stations further away from the entry point, it just means you are going to visit more places. It doesn't mean you are going to spend significantly less time trading. You'd shave just a few seconds in most cases, impact on time / CR ratios would be negligible.
As for the exploration... Would it really hurt so much if instead of "honk -> supercruise to a point A -> surface scan -> sypercruise to point B -> surface scan" you were doing "honk ->jump to a point A -> surface scan -> jump to point B -> surface scan"? Is it really so much of a problem that something in Elite doesn't take ages for once? It's not like you are doing anything worthwhile most of the time in supercruise (reading a book or watching something if the destination is far away - says a lot about the gameplay, really). Plus, you can be entirely sure that if the system is interesting, people would still supercruise for screenshots and general admiration of the visual side of Elite. I would be also in favour of sending out survey probes. But this again is a subject for a different thread.
