Apparently that's what Star Fox fans want. "Star Fox 64 in HD," they say.
I think you're harshly overestimating how many fans there still are to alienate. It may as well be pikmin as it is now. That's how antiquated this whole score based game concept is and why Star Fox sells less and less.
Well first of all, most people who played and loved Star Fox 64 back in the day have just forgotten about it and aren't Star Fox fans at all. They probably don't even know there's a new one coming out. Based on what I see from the actual fanbase, I've actually seen way more love go to the GameCube games instead of 64, which is just sad. But still, I think a lot of people don't want just Star Fox 64 in HD - even the ones who do like the core concept and gameplay of Star Fox 64 want way more than that for the new game.
If the score based game concept was so antiquated mobile games wouldn't sell as much as they do. I mean sure, it's a completely different situation, but it's still there to show it works. Star Fox Adventures sold half as much as 64 because it was nothing like Star Fox at all, not featuring any of the score based gameplay, barely any space shooting missions, and even lacking a major character until well into the game, plus being a borefest. Assault sold half as much because it came late into an unpopular console's lifecycle (an unpopularity I believe was forged by titles like The Wind Waker, Star Fox Adventures, Super Mario Sunshine and some others), and it was also an average game at best, and the whole arcade feel of it was completely lost. I never felt any incentive to beat my high scores in that game, ever - because it was too slow and unengaging in the Arwing missions and just... bad on the ground ones. And Star Fox Command sold half as much because it was crap.
But seriously, I don't think the whole score based concept is antiquated at all, but to be honest I don't think it was ever the reason Star Fox 64 was as big of a hit as it was. It's just a really popular IP. People love it for what it is. It's only natural that when Nintendo screwed it up over and over with the newer games it was going to sell less.