You can read a bit more about why the F-1 represents such an attractive aggressor to satisfy higher-end, supersonic-capable adversary training in this past piece of mine. In fact, going with upgraded third-generation fighters to replicate fourth-generation adversaries more efficiently has already led to one firm flying F-5s beating out another firm, ATAC, which had proposed using second-hand F-16s for the mission. You can read about the details surrounding this decision here.

Although this was just one particular contract, and there are many more with varying requirements, the fiscal value proposition that upgraded third-generation fighters offer may prove to be widely more attractive than the very high-maneuverability and other enhanced performance and capability features that true fourth-generation fighters bring to the table. This could end up placing even more demand on the aggressor Mirage F-1 fleet.