To me the gameplay on Zelda 2 is perfect. Any changes would make it worse.
The issue with Zelda 2, and Zelda 1 as well, is generational. Anyone who started playing Zelda when it was 3D thinks the NES Zeldas are wrong or primitive. But the truth is that the NES is actually when Zelda was at the peak of its popularity. On the NES Zelda was the #2 franchise. It has never been that popular since. Even on the Switch, with Breath of the Wild, Zelda is only #4.
Changing Zelda 2 would be like changing Shakespeare, because it's not like modern TV. It's perfect the way it is, but some people just don't get it.
I don’t think that’s the right way to look at it.
Both Switch and NES have sold 62 million units:
Zelda 1 only sold 6.51 million, and Zelda 2 sold 4.38 million; but Breath of the Wild has sold 18.6 million. To me that indicates that Breath of the Wild has reached more people and is therefore more popular.
I do agree with keeping Zelda 2 around in its original form - I tend to like the original adaptations of games myself, but I am also not against having an update of the game.
Even Shakespeare has been revised and adapted.
When doing comparisons it's important to note what the market was like back then compared to now.
There were 5.015 billion people back then, compared to 7.8 billion now for one thing. That's a 36% increase in people. So let's add that to the 6.51 and 4.38 million sold, meaning Zelda 1 would have sold 8.9 million now and Zelda 2 6 million.
But it's not just population growth. It's how big the gaming market was back then compared to now. NES was 62 mil, as you said, and (let's go conservative), at the low end Switch will sell 100 million. There are proportionally more gamers now days than there were in 1987, how many more it's hard to tell. But if taken into account, then those Zelda 1/2 would be much higher if NES had launched with this size gaming market.
Botw numbers, probably not, but maybe close.