Who would have won? Hot take: Brook would have been one of the hardest fights of Pacquiao's career, with his size, height, reach and jab.
Kell Brook? The guy who beat Porter 8 rounds to 4? His only career achievement? This content is protected Brook is a natural welter and even an aged former flyweight in Pacquiao would beat him. Brook's "Prime" 2014 would lose to 2014 Pac. Which says more about Pac than Brook really, Brook is very good but nothing special like his name suggests.
You can't seriously think that Brook would even actually be a competitive fight with Pac. We're talking about the same guy who lost to every elite level fighter he's faced and whose best win was Pac's sparring partners.
Prime for prime I'd agree, but it would have been an old Pacquiao if the fight had happened say instead of the GGG fight. That Pac could struggle against lesser fighters, e.g. with Horn who was not on Brook's level. Brook gave Spence a tough fight too and that Spence would have a good chance of beating an old Pac. I think the OP might be getting a bit too much heat in this thread for what isn't a crazy take.
Hard fight for the first half... I think Brook probably enters the second half up on the cards. But Pacs quality, stamina and power take over late and he wins a UD or late stoppage. Honestly wouldn't be surprised if he broke Brooks face down the stretch. But yes I do think it's a tough fight for 6 rounds.