How did the position open up for Fergie to be hired again?
Yes Ole got sacked, because results plummeted, but truth be told it was pretty clear he'd taken us as far as he could and should have been replaced the summer before. As much as I love Ole.
Why? What will that achieve?
You can't just give some 5 years regardless of results. It's a foolish idea, there has to be progression. Ron Atkinson won an FA Cup in 83 and 85 and never finished outside the top 4 but his reign was going nowhere. Should we have kept him on instead of hiring Fergie.
Fergie came and and showed progression, stabilized us in his first half season, finished 2nd in 88. And won the FA Cup in 90, Cup Winners Cup in 91 and challenging for league titles by 92. He only had one bad season really, but he had some money in the bank. Because he was restructuring the Academy behind the scenes. Contrast that to Amorim.
Klopp didn't need 5 years, he was challenging for titles, getting to finals and winning trophies within 2 or 3 years. There was clear and constant progression, in terms of results, performances and league finishes.
Arteta hasn't won a title yet and might still never.