Ice-types traditionally being late-game really boils down to the fact that Ice-types are, by nature, designed to best thrive in colder, snowy/icy conditions, and more often than not the icy/cold areas are accessible later in the game in almost every iteration of your standard Pokemon game.
Pokemon doing so really just follows textbook JRPG world design, as icy/snowy areas are almost always a late-game part of your traditional textbook RPG. A typical "RPG" adventure has you starting out in a more hospitable, human-friendly environment and exploring those kinds of environments first, with basic, down-to-earth and mundane enemies, and then less hospitable, harsher climates become areas that you explore later in the game, often these areas have stronger enemies and more "exotic", out-there, supernatural designs and powers.
An example would be that you hypothetically start out in a typical human-civilized village, and then your early areas you explore of the RPG's world are stuff like grasslands, forests, generic dark caves, and lakes and whatnot, and then as you head later into the game you start walking into deserts, treacherous mountains, snowy climates, and volcanic areas or dragon residences or something like that. Pokemon has traditionally largely conformed to this sort of world design, especially in past games where the overall intended progression was linear, and so as a result Ice-types tend to be found later and oftentimes have higher evolution levels as well, because the sort of environment they are designed to live in is something that is often only accessible in the later portions of the game.
*sigh*
Game Freak needs a trip to a cozy winter town where you can frolic in the snow and then go inside for cocoa. Snow and ice doesn't have to be dangerous and hostile. Route 1 could be a snowy route once, just once ... it wouldn't be dangerous.
At any rate, not more dangerous than any of the habitats they keep featuring earlier than the ice areas. Deserts, seafloor depths, poison swamps,
the inside of active volcanoes ... naah, any three-badge newbie can walk through those. But if there's snow on the ground, it's end-game stuff by default. Sixth or seventh gym at the earliest, which means you might be able to use an Ice-type for the remaining two Gyms and the Elite Four, provided that 1) your team isn't fully filled out at that point already, and 2) the Ice-type in question is good enough to actually fight those opponents. They tend to be slow and defensive with severe movepool problems, so you might not want to use them even if you can.
SV being open world raises the possibility that you can go to the icy areas of Paldea earlier than in past games, but whether you can use actually viable Ice-types in-game remains to be seen.
Sadly, it looks like the icy areas will be pretty much the last place you get to go in this game too. The icy area in Paldea is on the diametrically opposite side of the region from the starting area, and from the map it looks like it will require 'raidon climbing at least to access. I'm not sure if you'd have that from the start (like in, say, BotW) or obtain it later (like in PLA), but I fear that a beeline for the icy areas might not be entirely viable.
Still, though, a beeline for the icy areas should really not be necessary to obtain Ice-types. You will find Fire-types in other places than volcanoes, and Electric-types aren't exclusive to power plants. Ghost-types aren't confined to haunted houses. Plenty of Pokémon of pretty much every type are found as grassland creatures, but Ice-types outside the designated icy area tend to be annoyingly rare. Of course, Ice-types in general are annoyingly rare.
And I believe there is a bit of a vicious cycle going on here. Icy areas are designated for the late-game by default by the developers. They need Ice-type Pokémon to populate them, for the look of the thing. That means Ice-type Pokémon are typically designed to match the level curve of late-game encounters, i.e. two-stage evolution families that evolve way past level 30. This level curve design make the Pokémon unsuitable to be placed in the early parts of subsequent games. Sure, Swinub has stats somewhere between those of Pidgey and Rattata, but as it evolves at level 33 you wouldn't want to have it on your team from Route 2 onward. It would fall behind the curve fast, and stay there for a long, long time. Furthermore, since recent generations introduce relatively few Pokémon, there's not enough room for Ice-types that
don't match that late-game profile. And so, the available catalogue of Ice-types keeps being populated by designated late-game 'mons that wouldn't fit early-game areas. The problem is maintained and exacerbated by strict adherent to the same stupid limitations every time: Not enough new Ice-types to have some debut outside the icy area; no icy area before the sixth Gym or thereabout, requiring Ice-types to be tuned for the late-game; existing Ice-types are bogged down by availability restrictions or very late evolution; rinse and repeat every generation.
Not even the Dragon-type is being this badly restrained on such a consistent basis. And Dragon is an amazing typing,
meant to be hard to get but with a high reward. Ice-types are hard to get and usually quite terrible, due to their lack of good physical moves/special stats, terribad Speed, multitude of weaknesses, and lack of coverage moves. It's really not a typing worth putting behind so many obstacles,
every damn time.
