This chart shows the sales figures for Xenoblade, Fire Emblem and Golden Sun (in Millions). While the XB2 sales are slighly out-of-date, and I believe many of the results are from the notoriously iffy VGChartz, it gets across something which is often used against Golden Sun as a franchise.
For reference, Golden Sun 1 only sold 200'000 less copies than
Fire Emblem: Awakening; and that was back in the early internet era, when all it had was physical word-of-mouth, magazine reviews and a (not particularly good) commercial to get it out there.
To say it wasn't a success back in it's day is... well, wrong. Because if Golden Sun wasn't a success
then, then Fire Emblem: Awakening isn't a success
now.