Later on Tuesday, 7/9/21, McGowan is openly questioning any potential 'terror threat' with Basil Z. chiming in, both saying that no one has raised any issues of concern. You'd think the Premier would be the first to know if there was. That this came from the AFL leads me to this speculation: That Perth's going to put on a great GF show, that the prelim has sold out in a day, with both the prelim and GF promising bumper crowds. Perth running with the GF parade and showing up the rest of Australia - and in particular Melbourne - is likely too much for Gill and the Vics to bear. Hence, the cancellation of the parade.
A ticketed event in an arena? I'd say that would draw mainly the fans for both teams. But a parade through the city? That would be phenomenal and really well attended. Hence the major sook from the AFL.
I'd say if Perth does a whiz-bang job of hosting the GF, then this does their collective heads in. Just my .02 cents.