Right place, right time springs to mind.
As Brentford have shown, promoting from within is the right call when the model is established. You don't want an outsider to come in and upset the model/ culture that is working so well.
That is absolutely not where Saints are right now. As the debacle with Still showed, you need a big personality, gravitas and credibilty to make big changes. Or the players will simply ignore you- as we now hear happened with Still.
This feels very similar to what we needed when Koeman came in. Les Reed (for all his faults) said that they decided they needed a big name first and foremost and then used the data to pick the best manager available. There were other, more successful (in terms of data) managers available but he was what we needed AT THAT POINT. So the black box identified who was best within the parameters set. Not just.. whos won the most games/ got the best xG in Europe.
Now is the time for the same approach IMO. We have the resources and talent. We don't need a high risk choice to get some marginal extra gains. Basic competance with instant respect right now is the way to go. Viera or Rogers are the only ones in that category.