So I certainly could be accused of being an FX "fanboy" as I own two and love them, however I also have the unpopular opinion of not really loving the Impact specifically all that much. Why? From an engineering standpoint, you want everything to be as simple as it can be, but not one bit simpler. I feel the Crown, my favorite, already rides the edge of that line of too mechanically complicated. The Impact I feel crosses it, and we see that is one of its more common failure points, all the plumbing surrounding that plenum and transfer bar. And all that complexity does what? Puts the same barrel just a little further back so you get 6-8" more barrel length for the same overall length as compared to a more "traditional" rifle setup. Simply not my cup of tea.
However I think you're a little off-base for your disparagement of FX's barrel liner system. Glem above me hit the nail on the head regarding why you can't have all the same outer barrel parts for a given inner barrel; inner barrel wall thickness must be maintained in order to press in the rifling, so necessarily the OD can not be maintained as you go up through the calibers.
Let me give you a slightly different perspective on it though. When you're into high-end airguns, which lets be clear this forum is in a BIG way, there is a bit of a constant churn of trying to have the "best" which often translates to "latest and greatest." When you pay the premiums for airguns we do, it is very reasonable to want something which you feel is really as good as it can be. After all, nothing quite takes the shine off your new toy like seeing someone else's bigger shinier slightly newer toy. The important thing to remember though is that, just because FX rolled out some shiny new whizz-bang, doesn't mean your rifle now shoots any worse than it did yesterday. It is still just as great, and taking a step back to appreciate that can really help.
From an engineering and manufacturing perspective, I actually really appreciate what FX does. It is actually really really expensive and challenging to be constantly tweaking, modifying, and upgrading your designs. It means your back end parts support and logistics quickly become insane. Keeping track of all the different parts, and what works with what, is a real problem. A company which is committed to doing that is a real asset. Very few of FX's updates are a singular revolution, but taken on the whole they have built out multiple platforms now which are some of the most capable on the market. And that is fantastic! Selling all those parts as assemblies only...... a little more frustrating, but I understand it as it can reduce the total number of parts a given dealer has to stock. And that is also something to keep in mind, I bet FX is gets pushback from their dealers for doing this too, because they likely are also unhappy at the sheer volume of parts and model variants to stock, and when the latest and greatest rolls in, last year's model doesn't sell as easily either.
And there is one final thing about the FX STX barrel system I see people so frequently skip over: externally they're not special. If you think some other barrel is better, get it machined and fit it to your FX. How they attach and fit is all pretty standard stuff, so from Lothar Walther to BHW to Bartlein if you think someone else can make a better barrel or better price, you can just call FX USA and pick up the new caliber probe on its own, and put any barrel you want on your rifle. And I for one really appreciate that. Right now FX is, best I can tell, top dog when it comes to barrel technology. Their substantial investment seems to have paid off, in no small part because it allows relatively inexpensive and rapid iteration of new barrel designs. But this doesn't always have to be the case. Tomorrow someone could figure out a way to make a gain twist cut rifled barrel shoot some new ammo better than anything else, and because FX didn't make their barrel mounting system some form of eclectic proprietary voodoo you can just get that machined and drop it in.
Whew. Long post. Just my 2c, or perhaps with this long a post it'll have to be a whole 5c.