Lost traffic after rebranded

I`ve just had a chat with a business who lost all their web traffic when they rebranded and got a new website only they appear to have covered all the usual suspects. So what is missing on this list of what could have gone wrong: 1) 301 redirects not done or done badly 2) duplicate site 3) 301s done but content changed too much 4) coincides with a penalty There was a lot of old/archive content that didn`t come across (it didn`t get traffic) so they 301`d all those pages to the new home page. I`m wondering if this could have had an impact on the link profile once Google reviewed that? What else could have happened to make the site tank? It happened in May and straight after the new site went up. Disclosure: this isn`t a client so all I`ve done is ask them some questions and suggested they check GSC and ensure there isn`t a duplicate somewhere.