Argus morphed into Nexus, but the last Sandown show which was I think 2006, or perhaps 2007, was run by Traplet, who took that show on for its final year. It was poorly advertised at short notice and consequentially poorly attended. Traplet have since folded as many will remember.
There did seem to be a motivation to noticeably scale down some model exhibitions from the 1990's onwards. The original Model Engineering Exhibition seemed to gradually diminish in size and scope. It pivoted away from the now demolished Wembley Conference Centre, first to Alexandra Palace, then to Olympia, back to Ally Pally, finally to Ascot and Sandown race courses where the focus was solely on very high end model engineering.
I'm not sure what motivated this, money,risk or a lack of enthusiasm from the organisers, probably a combination of all three.
Having said all that many shows still exist, and some replacements came onto the scene. Wings and Wheels is one, and they put up an excellent pool, which is underused IMO.
There is the Southern Model Air Show, a bit of a misnomer IMO as the show encompasses all aspects of the R/C hobby.
There is also the London Model Engineering exhibition (assuming it returns at some point, hopefully 2022) and smaller regional shows like the St Albums exhibition which is well worth attending.