Yeah, except you didn't talk about WHY they are needed. Stuff like Ponyta and Larvesta's Flame Body are helpful perks, but it's not like those are what people use them for. Chinchou, Slowpoke, and Tirt have COMPLETELY DIFFERENT uses. The condition of being water types themselves doesn't provide a specific use of these Pokémon. When considering viability, there's absolutely no reason to consider that Foongus has other competition with Cottonee, Pumpkaboo, ect, simply because they all do WAY different things. These painfully vague and useless terms really don't have much place in viability discussions :l
People use pony and larvesta in similar ways which gives them the label "bulky fire-types" they both prevent Knock Off spam with Flame Body, they wall Fairy Types, bulky grass-types (yes that's a thing) they spread burn and the are quite good switchins to pawn because of the burn chance and being resistant to Iron Head. If I'm running a balanced team for example I may be weak to knock off, weak to Pawnaird, weak to fairies and think to fix this I will run a bulky Fire-type then I way the costs and benefits of Larvesta and Ponyta for example on my team. If I got hazards covered well and I need a pivot I will go with Larvesta, etc. Also a 30% chance to burn is great for Pokemon that have recovery and can constantly switch in to physical attackers
Bulky Grass-types are a similar role and they definitely can be compared in terms of costs/benefits to a team. Many bulky grass-types provide amazing support and are useful for sponging powerful water type moves from Pokemon like Tirtouga, Carvanha, Corphish, Chinchou. Say you need to have a switch in to these on your team, ferroseed, Cottonee and Foongus are your main options then in terms of your team you can compare which one is the best fit. If I have fighting types covered and I want hazard support maybe I choose ferroseed, if I like options such as Encore and Memento to help my sweepers I choose Cottonee if I want Foongus to have better coverage against fighting types and fairies as well as provide Spore as a support Then Foongus is the best for the team.
These terms don't have to be used, but Pokemon like these are often use for similar reasons with slight differences warranting them each spots on different teams I don't think it's fair you say these have no place in viability discussions when your sometimes post one liners regarding mons being discussed.
Also I am nominating Shellder for B+ there are many offensive teams that really on either Abra or Fletchling as there main method of Revenge killing sweepers and Shellder can get around both with multi hit moves ignoring sash and Ice Shard outspeeding Acrobatics after a Shell Smash. Although it doesn't have Sturdy, it's great physical bulk lets it Shell Smash quite easily and unlike other Water-types, it doesn't fear Foongus switch-ins because of Icicle Spear.
It's main downside is it's inability to get through steels if it's not running Ice Shard and with Pawnaird being quite common, this can be a problem. It does need eviolites removed / pokes worn down to sweep sometimes but that's like many mons I guess