What site are you using for your stats? Seems like we're getting different numbers. NBA.com is showing Yogi/Bogi at a -0.3 net rtg which still isn't as bad as I thought it was. Here are the 2 man lineups that played at least 500 min together. People hate Willie but he was part of most of our best 2 man lineups. Also not that it means a ton but Cory Joseph is in the top 2 lineups if you switch the team over to the Pacers.
https://stats.nba.com/lineups/advanced/?Season=2018-19&SeasonType=Regular Season&TeamID=1610612758&GroupQuantity=2&sort=NET_RATING&dir=1&CF=MIN*G*500
CoJo has had a VORP between 0.5 and 1.0 for like 6 years straight. His defensive box plus/minus in that span is always somewhere between good and very good. The only players on the team with a negative VORP were Ferrell, Koufos, McLemore, Skal and Mason. Ferrell played twice as many minutes as the next closest in Koufos. He had plenty of time to play better and just didn't. He also didn't facilitate very well. So he comes in and it causes Bogie to be the primary handler and now he can't even use his main skill (shooting) as well as he should because he doesn't have a guy that's good enough to pass him open frequently and he doesn't have the skills to get his own shot very often. Top it off with his defense, which like you said, isn't IT or Mason level but it isn't CoJo level either.....and now you have a player who just isn't very effective on this team.
Last year CoJo was something like 4th in DRPM and Ferrell was 88th or so. I just think in our specific situation, he's a better fit. I would have rather had Satoransky at 30mil over 3 years but that didn't happen so I'm ok with Joseph even if he's overpaid. I think it'll show when the season starts and he covers up for a lot of the negatives that Yogi isn't capable of covering.