Did you play back in 2006-2007? These were the graphics, my friend. It wouldn't be Old School without them. If you want new and improved high-quality graphics, I would recommend playing RuneScape 3.
This has been asked for many times and never been that well received.
I mean its called old school runescape for a reason.
Your idea like the veteran fourmer stated has been around for quite sometime. There was even talk of a 3rd party client bringing back the 08 graphics but this was denied. The reason for the denial was because Jagex's botting systems could detect a third party using a system to change the graphics into HD. They looked into this and put a no on it for the time being.
I would not assume they would make any graphical update. People would be very disappointed.
Correct, as I said, they put a no on that for the time being. They said they have no active plans to create RuneScape in HD. They offered a 3rd party client a chance to provide them with the source code so you could convert Old School into RuneScape HD, but the client did not offer their code to Jagex for review. Therefore, what killed your idea was their cooperation with Jagex.
RuneScape 3D was a major update to the game and the Old School team is way too small to conduct such an update. Overall, I do agree certain aspects of HD was good, I do not see it being implemented in the future.
I wish you best on your suggestion though.