Here's my thoughs: I have a feeling that because of the GC line, the demand for more modular stuff will increase, and sales for the modular stuff will increase as well.
Think about it: There's lots of us here who have been on tone quests for years, if not decades. And our search ended with Egnater. And while the GC line of stuff is absolutely incredible, a lot of us with $3K+ worth of Egnater Custom shop modular stuff just don't have the need for another head.
That being said...
Guitarists are constantly upgrading. From the day you get your first amp, you're thinking about your next one. The more affordable GC line is introducing the great Engater tone to a whole new demographic of players searching for an amp at a different price point. And when it comes time to upgrade for them (if necessary), they might very well be inclined to stick to the brand they know, and upgrade to the modular series.
Think about how many Marshall Loyalists are out there. "Dude, I got my first Marshall combo when I was 12, and for me, it's been Marshall ever since...". It could very well be the same way with Egnater for a new generation of players. I think as time goes on, and perhaps as their paycheck increases, lots of the fans of the Rebel and Tourmaster will be inclined to take the step up to the Mod50 series.
And as more and more people do so, there will be more opinions, suggestions, and demands for more modules.