From Zero to Hero, Metagross
I never thought we'd see the day where a steel type is the third best attacker. Metagross used to be that pokemon that was rare, but not worth the cost of evolving and powering up because of how bad it was, even agaisnt the 2 regies, it was largely a joke of an attacker when compared to other easier to obtain, and more useful pokemon.
Today however, that has changed. A chart topping attacker with the fantastic meteor mash, boasting incredible neutral DPS, impeccable bulk, and a type combination thats amazing for both raids, gym offense and gym defense. Now that its got the moves, it actually can hold a gym well and threaten attackers. On offense, its a fantastic counter to poliswine, who with the release of Mamoswine and its fantastic ice set will likely become a high priority raid, and its massive bulk with great neutral damage will serve incredibly well as a gym generalist, not being able to be walled out liek machamp or dragonite agaisnt gardevoir (who metagross has the advantage over compared to the other two's double resistance agaisnt), likely will earn a spot on many gym taking teams
Metagross might not have too many legendry raid match ups in the future, but rarely have community day event pokemon been current legendary coutners (though it will be interesting to see what it can do against deoxys).
Answers
I think you're oversimplifying this by quite a lot. For one, Swinub is biome dependent and some people already have enough candy to obsolete Weavile and Glaceon. For the people who do not have enough, doing tier 4 raids is a significant cost, you can't do it alone but most raiders are only interested in grouping for legendaries.
And if you rely on raids for Pilo/Mamoswines, you might get Glaceons without as much dust investment because WB Eevees can spawn in most biomes, so at least that's a niche for them.
I was saying with Piloswine being a raid boss once more, and with mamoswine being as good as it is, piloswine will become a higher priority raid boss
NEver said it was hard, but Meteor mash metagross out DPS's kyogre by around 3 DPS. Its a better counter to anything Steel and water have the same effectiveness agaisnt than kyogre, and the same applied for steel and fighting and ground with machamp and groudon respectively.
