It is true that one needs an sct e.g. for small galaxies or planetary nebulas.
But still, many dso's can be photographed around 1400mm. And for dso's one wants a speedy system.
Many dso's are simply too dark for an f10 scope, although that changes with every cmos generation that is more and more sensitive.
I admit that I still have to try using the edge more often natively and with a higher gain number or a more sensitive camera than the asi1600. Maybe f10 is then more usable than at unity gain?
The problem is not so much that the native edge 800 has a long focal length.
It is that it has f10 and not f7 or f5.
One now has things like the rasa. But that is a dedicated wide field concept.
Ideally, One would like something at 400mm for really wide fields. Such systems exist en masse (Celestron has the rasa now, and one can use affordable flat field refractors)
Then I would like something at 1200-1400mm for middle sized dso's at f5-8 (for which celestron only has problematic focal reducers with coma problems)
and one would like something with 2000-3000mm for really small planetary nebulas and small galaxies. At best this should also have f<10 but that would become untransportable. For that, Celestron has the edge hd line.
I personally use the edge 800 for small things, but for really small planetary nebulas, one needs probably an edge 11 or something...
The problem is that we do not really have something unproblematic at around 1400mm focal length.
The refractors in that area are extremely expensive.
If I use the 190mak newt from skywatcher, I get collimation problems. I would get spikes in the image and I would have to rebuild the scope basically.
An RC will give me the same collimation hell and create even stranger spikes.
So I think there would really be a market for a telescope of around 1400mm and f8-f5.6 which does not produce spikes and delivers the performance and problem free handling of the native edge 800.
At their native focal length, the models from the edge series are absolutely fine telescopes.
I am not totally sure what point you are trying to make. If you want to image at 1400-2000mm FL with a 8-10" scope then by definition it will be a slow scope. If you want a fast focal ratio long focal length scope it will start to get big / unwieldy very fast (not to mention expensive).
I think there are some misconceptions around F ratios and optimal sampling which are hangovers from CCD imaging and continue to persist. With low read noise cameras F ratio is less relevant and the impact on SNR is much less.
The EdgeHD 8 paired with a small pixel highly sensitive camera is sufficient for most small objects. I use the EdgeHD 8 at F7 and F7.5 with my ASI183MC & MM all the time for imaging smaller objects e.g. galaxies, planetaries etc. It produces superb SNR and detail. I love imaging galaxies and have never felt the need for a larger scope. For larger objects I pair with either the Redcat51 or my AT60ED. Delivers lovely high resolution wide fov images.