Allen hasn't been good, but the failings here are more on the Saints than on him. They went all-in on the end of Drew Brees' prime (a smart move IMO), but then after Brees retired, they kept trying to duct-tape their roster together rather than tear it down and rebuild. The 2024 Cowboys are basically what the Saints have been for the last 3 or 4 years... limping along to .500 finishes with bad contracts maxing out their cap.
Derek Carr is the epitome of this decision making. Just the kind of spit-and-duct-tape quarterback who's functional enough to keep the Saints from collapsing, but he's too mediocre and too expensive to take them anywhere. And now that he's hurt, it's all unraveling.
As far as Allen, he's your classic Peter Principle coach who's been promoted one step above his head. A proven DC who's a crappy game manager at HC. After two failed HC stints (the other one with the dumpster-fire Raiders, which wasn't really his fault either), he's just settle in coordinator and staffer work for the next decade. Hell, he is just the kind of retread DC that Jerry likes to bring in.