Anyways, my answer to the question of the thread is no, any theoretical New Leaf HD game wouldn't have sold as well as New Horizons. The biggest reason why is that it would've just been a remaster of New Leaf at its core, regardless of whatever changes were made. We've already seen this play out with Wild World and City Folk. Wild World was pretty well received by the fans at the time. Not without its flaws, of course, but it was still pretty well regarded and enjoyed by people at the time. City Folk comes out, and generally it's not as well received as Wild World. Why? Because it essentially was a copy of Wild World. It did make changes and added some things in (like the city location), yes, but it was widely seen as just "Wild World 1.5", and you can argue the sale numbers reflect that (from 11.75 mil with World World on the DS to 4.32 mil with City Folk on the Wii). And that's what we would've seen once again in this theoretical scenario. Some people have complained about New Horizons, yes, but there would've been even more complaints about New Leaf HD not changing much from New Leaf and not really being justified to be released as a new game at full price on the Switch when the original New Leaf was still pretty accessible at that time and cheaper to boot. The end result would be New Leaf HD dying off much quicker than New Horizons. It still would've sold alright, but it would come nowhere close to the amount of sales New Horizons has had.
The
WW/
CF analogy is strange, because the latter didn't just add/change things — it outright
removed content from the previous game, and I personally disagree with the notion of it being "
Wild World 1.5". It's more accurate to call it an amalgamation of both the GC game(s) and
WW. It removed the fictional events from
Wild World, but brought back the RL-based ones from the original game, and despite still keeping the Rolling Log mechanic, the town layout is structured more similarly to how it was in the first game. Villagers still had birthdays, but they didn't celebrate them, anymore, and dialogue options were stripped out entirely, save for favors and such; villagers didn't even have photos of themselves to give you, anymore, and there was only one Episode you could go through
once per player, and it's in service of a then-new character.
City Folk rightly deserves criticism because it plays it safe without improving upon the
AC formula much, rather than it just being a "clone" of another game — which is a sentiment that frankly doesn't make much sense, in hindsight, because it ignores the actual problems of the game; issues that didn't exist in the first two games.
Also, the OP isn't about an HD
New Leaf remaster. It's a hypothetical question of how the game would've sold
if it had been developed as an HD title with
New Horizons' mechanics in mind,
instead of what it was (and is) when it released on 3DS.