Something being able to come in on the hazard setters and other pokemon and get off defogs, does indeed make them a reliable defogger. I didn't mention any sets for them, refresh my memory if I did. I prefer not to spoon feed, providing the slither wing option is already overstepping my generosity in that department as people tend to be too lazy to explore other options. What are you talking about, talonflame is indeed a notorious magic guard user as it uses it to fire off recoilless moves and come in on hazards. If you call removing hazards, halving attacks, or getting KO's and having longevity useless ... I do not really know what to tell you. Brave Bird and Flare Blitz are 120 Power moves, they are indeed doing damage when used by a pokemon with no EVs. You are attempting to downplay something you admittedly do not use. "it can technically run willow-wisp" ... the word technically does not need to be there. People are not using many old mons because once again this is a new gen and everyone is experimenting. You keep using the word useless but I do not think it means what you think it means. Magic Bounce does not get rid of already set hazards and one pokemon cannot check everything.
You do not seem to comprehend that pokemon can be used for various reasons and purposes and are not just limited to what you Google or see on YouTube videos. You say they lack bulk, yet I do not any world where a hazard setting tusk, treads, tien-lu, gastrodon kills Hawlucha. You say the term defog bot yet Corviknight in his essence is a defog bot, you switch it in on hazards setters to try and stall out their hazards, on things that aren't obvious nukes, and on certain choice locked moves you expect to happen. Skeledirge isn't a prime good as gold abuser, the set farther above was just made as an example and they thought it walled corviknight, and someone had to advise them to use lick of all moves. Its bulk is nothing to something that would come in and kill it, it's not a perfect pokemon those being. Riiiight, palosand why by your same logic doesn't exist.
Knock is there ... on primarily the hazard setters, the things you switch the birds in on. Corviknight isn't the only option, it's the most common option used, one that everyone expects.
Corv Defog bot??? Literally the most common special wall in the tier + slow pivot fym Corvknight is a defog bot. And that set isn't just an example, it is the set I've seen most commonly on ladder, for a GaG abuser. Also non-invested Talonflame even with the power of 120 BP isn't doing jack to anything defensive, case in point 252 Atk Talonflame Brave Bird vs. 252 HP / 0 Def Ting-Lu: 114-135 (22.1 - 26.2%) -- 9.2% chance to 4HKO and that's a non-defensive Ting-Lu with an invested Talonflame while Ting-Lu has no physical investment. Also like I said, it isn't a "notorious" user this generation because literally no one is using Talonflame (for good reason), although I'm not saying it wouldn't like MGuard, hell I used MG Talonflame on probably half my teams last generation. This isn't countering the fact the mons and sets you bring Defog on will end up having no use other than being a Defog Bot. Hawlucha isn't doing much offensively with its pathetic 92 attack and limited moveslots with Defog/Roost and its speed isn't great this generation either, same case with Talonflame although at the least that can get more usage with Wisp. Even setting up with those mons doesn't do much as you will still do little and get Whirlwinded out. Skeledrige has fine bulk to switch on Corv the most common Defogger and can take a Treads EQ and threaten with Torch Song, and if a mon comes in to revenge, well it can leave easily since its job as a GaG mon is already done, you've forced the Defogger out. Also yes bulk is important for a defogger, you even acknowledge that partially in your opening statement as Talonflame and Hawlucha has little bulk to survive throughout the game and ever come in anything remotely offensive.