I think you might be on to something with your assertion that Fleet Carriers were kind of thrown out there at the expo.
But I don't think they were created for the expo.
I think Fleet Carriers might never have been intended to be space winnebagos, but instead were meant to be flyable capital ships that required multiple people to crew them to be flown. This would have been targeted as New Era stuff.
So basically, squadron ships. Starship Enterprise type stuff. But maybe somewhere along the way this was determined to be too difficult and perhaps too niche and therefore canceled, which resulted in them being announced at the Expo as something else.
However, time passes, and rather than release something that might be perceived as lacklustre; and perhaps also wheels have been turning behind the scenes and intent is now to try to capture more of this original "New Era"-style vision for them, or lay a foundation that could possibly be realized later post-New Era-1, they are being witheld.
I gotta say though. If you were Frontier, and you had this major update cooking, and you were looking around and saw you had currently the only space game (I don't think Bethesda's thing is ready), aside from NMS, which already kind of had its day (but could very well possibly do the re-release thing too I guess), and you were staring at that future console-generation launch, you would want to hold it all for that, too. In fact you'd probably feel stupid for not doing that.
