If you update his design... then he stops being classic Sonic. This is a classic Sonic game. To the vast majority of Sonic fans (i.e. the ones working on this project), something like that matters a ton. We're talking different proportions, different colors, a different character design entirely.
What you are proposing is almost like saying that if Megaman 2 was remade in HD, it should use the Megaman Battle Network designs. The Sonic fandom is split up in to specific groups of people who like the different interpretations of Sonic over the years, from Classic Sonic (Sonic 1-3), New Sonic (Sonic Adventure), not to mention the different interpretations of Sonic displayed in the three cartoon series (and each one of them has a different personality and tone), the Sonic X anime, the Sonic OVA (aka Sonic the Hedgehog the Movie).
There are a lot of different ways to portray Sonic and getting it right for the time period is extremely important (especially when you consider there are hundreds, maybe even thousands of people who prefer 1992 Sonic over today's incarnation), otherwise you might as well not even remake Sonic 2 in HD at all - you might as well just make an original Sonic game.
