His contract extension alone albeit strange (many said that at the time) was not a disastrous decision, but it showed a glimpse into the club thinking forward. The other way to look at a single decision is to say that that is the reason we won the title that season, which would be equally wrong.
I think that the decision to keep Ox long-term indicates that we had no plans to strengthen the midfield beyond the next couple of years. We then lost Lallana and brought in Thiago (I think). We had Shaq (blowing hot/cold), Ox and Keita often injured (the latter more so), Curtis promising but not established even now, peak Hendo and Gini, who raised Fab's game... It was all dandy. But with Gini gone, Hendo and Milner needed succession planning at the very same time that we need Ox / Keita / Curtis succession planning. And I don't always agree with Al, but he's spot on when he points to who we bought in that time, even young talent - non-midfielders in Elliott and Carvalho (Diaz, Jota were needed for different reasons, and I'll leave Gakpo out of this).
That's how I see Ox's contract extension - if it wasn't offered, we'd have another midfielder lined up for 2020 or 2021. And we needed that.