Smith, like Rosler and Warburtom before him, and Frank afterwards, took on the Brentford job and left them in much better shape by the time of his departure.
He then did the same with Villa - a good deal nearer the bottom of the Champiopnship than the top when he arrived, got them promoted, then established them in the PL. And sure, they've had a bad patch recently, but what does anyone expect? Straight to the title without a single mis-step?
Fact is, Villa's owners are thinking top six, maybe even top four and soon. And while there's nothing wrong with ambition, like many other owners who are chasing a target, they don't seem to realise that it's a moving target i.e. no matter how much they improve to get to the (current) level of L'pool, Chelsea and Man City etc, by the time they get there those clubs will likely have moved ahead, too. Meaning it's going to take time - maybe five years?
Now I happen to think Gerrard will be a good appointment, but in the end, he'll either improve them or he won't. If he does, he'll be off to an even bigger club beforethen (L'pool if/when Klopp leaves?), or he won't, in which case he'll get the sack and Villa will be looking for his replacement.
Imo it would have been wiser to wait another while to see whether Smith really had taken them as far as he could, in which case Gerrard would likely still have been available. Or if not, then some other (more established) manager could have been sought.
Otoh, Smith might have pulled them out of their present problems and they'd have avoided all the present instability.
All of which reminds me of the old adage that theres no problem in football which can't be solved by sacking a manager. Which is why Rochdale are Champions League winners, season-in, season-out.