4990 in output quality, and hardly much to V700 and by extension V800.
The V700 has a remarkable optic improvement over the 4990. Still many shots are not sharp enough to challenge the 4990, many shots are hand held and focus is not always perfectly nailed, also many scenes are in the DOF and not in the ideal plane of focus, and film recording capability has limitations in many situations. Still for a BW grain structure depiction we may want a lot of resolving power.
The 4990 has a single lens that has to focus just on bed (for opaques and 8x10 film) and also it has to focus at the holder's height for the rest of the formats, with the same single lens.
Instead the V700 has two different lenses that are automatically interchanged depending on if a holder is detected, one lens is covering the entire bed and focused to it, and the other cover 5.9" delivering well more effective resolution (specially when scanning in holders), and it is focused to the film height in the holder.
V800 adds LED illumination that requires no calibration over time and requires no heating delay. Also new V800 bundled holders (that can also be purchased/used in the V700) have ANR glass to ensure flatness and height is adjustable to nail focus, beyond speed improvement. Spme people don't like the new V800 holders because they find fringes at max pixel peeping, Personally I don't have that problem.
It appears, despite much superior technology being available, nobody bothers changing much
Sure... Market size and lack of competition are not helping the innovation.
Still the scanners that resisted have a great value... We are fortunate that Epson and Plustek are still active in that niche and sourcing new gear, those are not totally Pro scanners, but if a wise usage is performed they can deliver totally Pro results.