Eventually there would have become a point where if they had not remastered TOS into HD, TV stations would have stopped buying them. Maybe not today, maybe not in 5 or 10 years but at one point in the future. So some fans of it today might not like it, but is it not better to preserve it so that future generations can enjoy it.
And they did try(for the most part) to keep the shots similar to what was originl filmed. They could have gone a lor further and there is a certain segment of the fan base says they should havve gone to town with the updated effects same goes for TNG. It might be nice to see something other than Miranda/Excelsior Class ships, or re-use of movie models for freighters.