Well yeah Keane left Sunderland 18th mid-season but it was him that got them promoted and kept them up the previous season in the first place. Ipswich were just a **** club that couldn't attract or keep the players he wanted.
The managers you mention..
Pochettino took Espanyol bottom of La Liga and was sacked. Simeone took River Plate, Argentina's biggest club, bottom of their Primera Division and walked out. Then there's Conte and Klopp - both relegated and sacked in their early careers. Gattuso was sacked twice at lower tier clubs before getting the Milan job. Even Ferguson finished one place above the relegation zone at St Mirren and was sacked.
So you know, I'm not going to rule anyone out for not setting the world on fire at Sunderland or Ipswich
With Keane, the case isn't about his record but the character and values he shared with Fergie and which match United. He'd bring back the winning mentality, motivation, confidence, fear factor, standards, attacking football... the United way!