Sure TOS *could* have been Blu'd as was, but that doesn't mean anybody would have bought it. And nobody would have. So from a marketability standpoint, TOS needed its bath way more than TNG does.
TNG needs new FX and that's really about it. The level of color adjustments/enhancements and scene by scene detailing that went into TOS will never go into TNG.
At this point along CGI's evolution, FX isn't that expensive anymore. It's all the other stuff, the pain staking scene-by-scene stuff, which is expensive and that's what TOS needed a lot more of.