No guarantee, sure. But in football, as in business, you look at trends. There was no guarantee that Moyes wouldn't suddenly lead us to a PL win the next season, but the trends were clear. The team was struggling and regressing and so he had to go. So, what's different now? Nothing at all. The trend is that the team is clearly regressing and is struggling. How many examples are there (again, barring Fergie circa 1989) when a manager has struggled so badly and then suddenly turned it all around to lead his team to unprecedented success?
Regarding the Moyes issue you mentioned, I think the fan's tide turned almost completely when he did the "but I wasn't worse off from last season" and "even SAF would have struggled" quotes, but the board only fired him when it is mathematically clear that top four was unattainable rather than the case they woke one day and recognised the trend was downhill.
I'm a consultant and so shall give you a business analogy. At times, companies appoint an external CEO. Each time, the CEO is carefully chosen by the Board. Many on the Board know nothing about the company's actual business, but all have a sense of finance and can assess CVs. Each candidate has exceptional qualities and one gets chosen. If he fails, the Board moves and appoints a new CEO. The sacked one may go on to lead other companies to glory and all that proves is that he just wasn't the right man for this particular company. His half-season at Chelsea hasn't made Jose a poor manager, just like his stint at United doesn't wipe away LvG's past glories. It's just that both were misfits at this juncture for their respective clubs.
I like that you are bringing in your perspectives as a consultant; it provides more than just a fan's perspective. I think it is very fair to say that both managers seem to be misfits at this current point. Though maybe Mourinho's track record may be more relevant to modern Football, if there is such a thing.
In a similar vein, I am a commodities trader and read all sorts of media spins and see all sorts of conflicting statistics regularly. Even if your analysis is right the markets can prove you wrong. We deal with uncertainty and irrationality Everyday. At the end we are judged simply on the Mid-year and final year profits (and losses). That's more of where I am coming from, do I stop the loss that is LVG? Many fans think so, I don't disagree. But I have had losing trades which turned out to be profitable ones if I hadn't cut them and others that made it genius to stop the bleeding early. So maybe that's why I am rather ambivalent about firing LVG now or end of season.
As a side note, I am just wondering how Ed's conversation with Glazers will go.
(The below is pure idle imaginative speculation)
Ed: So I propose that we sack LVG now.
Board: Are we out of the top four?
Ed: Not yet. Season's only halfway. But the signs aren't good. Fans are complaining.
Board: fans always complain. How are our merchandise sales and match attendances?
Ed: still ok. No discernible decline.
Board: So who have you got in mind to hire?
Ed: Jose Mourinho.
Board: thought he was the manager of Chelsea?
Ed: He got fired recently. Due to bad results at Chelsea who are sitting around relegation zone.
Board: so you want to fire LVG now and replace him with a manager that took his team from champions to relegation?
Ed: oh but Mourinho's got a good track record.
Board: I thought that was one of the reasons we hired LVG.
Ed: ... ....