Hello everyone and thank you for your answers. I sincerely appreciate it!
I didn’t follow the redesign phase, I’ve just jumped on board now so I actually have no idea why they didn’t go for the 301 solution.
As Monica pointed out the 404ed pages were actually valuable pages and, at least in my opinion, this is proved by the fact that now their traffic is close to 0. Their traffic literally dropped in a matter of days (kind of scary to see such a steep fall). I agree with Travis when he says that just the valuable pages should be 301ed, but the thing is that they sell their products online, meaning that hypothetically every (product) page is equally important. They were neither old nor poor quality…I guess they just skipped the 301 step. I will do some more research but I guess that, as you guys suggest, the best way to go is 301 all those pages and see what happens.
I have no idea if they did anything on the social side but that’s worth investigating some more.
Thank you very much for now! I will keep you updated 
Cheers
