1) IVs matter a LOT in gyms/raids. Poor IV Salamence won't last nearly as long as a decent or high IV one at all. I've got a 56% Dragonite that just gets demolished by anything and doesn't put out much damage compared to my 84% that actually does a good amount of damage and can tank noticeably longer.
This really is a completely separate argument, and one I am not well-prepared to debate for. All I can really say is that a powerful Pokémon like Dragonite or Salamce can still be good battler at high levels, even with average IVs. Sure, a 50% won't be as good as a 100%, but it's not worthless either.
Plus, I literally said they ALREADY have blockers for distributing powerful Pokemon so easily. You can't receive Pokemon with levels higher than your trainer level, CP and IVs are also randomized.
The CP only changes in accordance with the IVs. The Pokémon's level remains the same, if the trainer receiving the Pokémon is a high enough level. And even if the level is lowered for a lower level trainer, it will still have the maximum power that that trainer can have, making it a powerful Pokémon for them. Sure, a level 30 trainer can't get a level 40 Pokémon, but level 30 is still powerful, and they're still saving effort, candy, and Stardust by doing the trade.
2) Spoofers don't benefit AT ALL. Literally how is it hard to realize that "Oh, I can get regionals and rarer Pokemon much easier now that I can trade with anyone anywhere." It would destroy the supposed 'spoofer black market'. Why would you contact a spoofer to trade when you can literally go online to forums like this and see if anyone has the Pokemon you're looking for and if they're willing to trade? It'd literally be no different than version exclusives in the main series games or trading with people worldwide in the main series games. I don't understand where this 'fear' of a spoofer black market comes from when you could literally trade with anyone in the world rather than just people 100m away from you. Spoofer black market is more likely NOW than it would be with worldwide trading
I'm literally saying the spoofer black market fear is moot because you can literally trade with anyone, anywhere. Only people with half a brain would contact a spoofer to trade rather than get on a site like Reddit or even here.
The thing is, you have no idea if the person you're trading with is a spoofer or not. All you'll be doing is looking for someone with the Pokémon you want, and that will give you the best deal. And spoofers will naturally have a larger supply of Pokémon worth trading, and because of how easy it was for them to get it, they'll likely trade it for cheaper than someone who put a lot of time and effort into catching/training the same thing. Whether you know it or not, spoofers and botters would greatly benefit, and people would end up with illegitimate Pokémon without knowing.
As for the black market, there will always be someone who wants some rare and powerful Pokémon, but has nothing to trade for it. Some of those people, for whatever their reason, will be willing to pay to get those Pokémon. The market will be there. Maybe it won't be gigantic, but spoofing and botting players will be making money off of it.
3) There's more socializing in getting to know dozens of people around the world than there is walking up to someone you already know to trade for Pokemon you probably both have anyway.
You're not really getting to know people when all you're doing is telling them what you want and what you can give in return. At lest with the current system you're actually talking to them. Besides, the type of socializing Niantic cares about is real-life interactions, not virtual ones.
4) How would allowing people to trade worldwide cause people to spend less time on the game? If anything it'd make me spend more time trying to catch Pokemon to trade with more people. Right now it's just not worth it unless it's for Pokedex entries.
I've already said why. Online trading would be so easy that it would easier to browse trades on the couch than get out and actually catch stuff yourself. When given two options, people tend pick easier one. Yes, you would do some playing to get stuff to trade with in the first place, but in the end, you'd be only doing the minimum needed, so that you can save yourself as much effort as possible.
!!! The daily limit sucks though 
