I don't get the impression that the FX8 was all that much of a success from a business standpoint. The relatively short period of active production points to this as well. It is a great product and I am happy that I have one, but I would be suppressed to see Fractal to make another tube amp oriented FX only product again.
The fraction of the market interested in a professional quality digital FX unit optimized for use with tube amplifiers is likely very small. Most tube amplifier purists want a traditional pedal board and will not buy a digital device with menus, a computer based editor, and firmware updates. Most of the players interested in the advantages of a high end digital effects processor could be easily convinced to transition to a fully digital modeling based rig.
Tube amplifiers are unmatched in raw tone quality. Digital modeling asymptotically approaches (will never EXACTLY match) the performance of analog circuits, but the Axe-FX III / FM9 / FM3 all sound practically indistinguishable from a mic'd amplifier in a blind comparison. Once you factor in the lower noise floor, reduced cable management concerns, superior reliability, consistently great sound, smaller size, lower weight, and the unparalleled tonal flexibility that modeling offers, it is easy to argue the case for a fully digital rig (especially for live use). It was the build quality, extensive flexibility, and great tones from the FX8 that pushed me over the edge to try the Axe-Fx III. Now I play the Axe-Fx 98% of the time while my tube gear goes unused. I play through my amps occasionally but honestly find the Axe-FX III to be equally (if not more) satisfying to play. If you want the sound that you get with a tube amp then get a power amp and cabinet to go with your Fractal modeler and you're set.