I don't think Smart is falling off, in any sense, unless you mean he hasn't won an NC in four years....
Maybe he peaked for two years and now is back to normal, top 10 finishes. No team seems likely to be as dominant as some preNIL. It might happen on occasion by luck.
To be fair, I've never been one to claim Smart is a generational coach, I think he's very good obviously in most areas. Their recruiting rankings this year seem to have dropped off, if that matters.