I feel that the whole "SC is doing well relative to actual completion rate of FC" perspective, while valid, is too defensive to the position of the reality that SC ultimately doesn't have a very strong retention. Or to say, it means that only the hardcore small percentage of completionist actually bought SC.
Or to put it differently, SC has zero appeal to anyone except the ones who finished FC. I would consider that to be worrisome.
Going by historic games completion trends, many gamers fundamentally don't finish games and if that's the case, then sequels are forever doomed to never sell better than 20-30% of what the first game did. But many do, even when they're direct sequels or core to the story. The purity of a story's progression thread from game-to-game... is not as important to many gamers as we would imagine ourselves to be, given that we are core Kiseki fans and deeply invested in the lore.
I'm more in line with duckroll's view that FC's success was buoyant a lot on the value perception of it, the timing of its release, and that it also benefited from the first-mover advantage for the Trails brand during its first release.
Honestly, it's a tough situation to be in. It's hard to think what would be the best way to elevate Trails above this situation.
From a Trails fans perspective, the simple answer is of course, localise 3rd, Zero, Ao in good succession with enough window for all games to breathe, localise CS3 as soon as possible, and try to maximise the benefits of it being the first seemingly high-production value Trails game on par with a Tales game, and hope that a PC ver of CS1/2/3 can happen.
But honestly? Looking at SC's sales, I wonder if that's truly the best choice from a business sense.
Practically speaking, if I had the power to convince Falcom to make one PC port of Cold Steel series happen, I would ask for a PC release of Cold Steel 3, and then put in all the effort possible to sell Cold Steel 3 as a RPG worth paying attention to by your broad JRPG audience on Steam and PS4, and make that game the gateway to the Trails brand by combination of two modern platforms, a product that appears relatively high quality to the JRPG market, and I'll pull the Mass Effect 2/3 thing of "y-y-you can totally jump in with the third game in the series!!!"