I think the kicker with The Golden Company coming into play is that it has come at the hands of the Lannister army. What's more, it's come at the hands of the grain - food is becoming a huge priority across Westeros, we've heard Sansa discussing it twice in two episodes and now it's been discussed in KL. Robert's prediction came true with that.
Let's say Viserys Targaryen lands with 40,000 Dothraki screamers at his back. We hole up in our castles. A wise move. Only a fool would meet the Dothraki in an open field. They leave us in our castles. They go from town to town, looting and burning, killing every man who can't hide behind a stone wall, stealing all our crops and livestock, enslaving all our women and children. How long do the people of the Seven Kingdoms stand behind their absentee king, their cowardly king hiding behind high walls? When do the people decide that Viserys Targaryen is the rightful monarch after all?
Noone can stop the horde, particularly with the kicker of three dragons on top of it.