Why 80 instead of 60, or even 50 megapixels may be a more relevant question considering the available options. If you like high resolving power 80 megapixels is the current best (except for stitching) so just go for it. However if you have economy considerations, or lens compatibility considerations for tech cams a 60MP back is probably wiser option.
I'd like to have say 400-600 megapixels or so to really outresolve the optic system, make sure all digital artifacts are nullified by diffraction. That's not available today though, and today I don't think it's too big a difference between 50 and 80 megapixels.
To me making a print look nice big is not necessarily that the eye is out-resolved on nosing distance (although that is cool), but that the print lacks ugly artifacts on nosing distance, and what's ugly is a matter of taste. For example I prefer the structure of film grain much more than digital jaggies.
I prefer an oversized digital print look fuzzy at nosing distance rather than having false detail. I don't like the look of fractal upsizing either, I rather have that standard bicubic blur. Previously I shot at f/11 most of the time, but nowadays I shoot at f/16 with my 6um pixels (50-60 MP backs). Why? To reduce aliasing artifacts and give oversized prints a smoother look up close. May sound backwards to reduce resolution by diffraction to print larger, but that's exactly what I'm doing. I've done my own tests and come to the conclusion that I prefer that nosing look, and welcome the increased depth of field. Of course on normal viewing distance it doesn't matter, resolution is fine. Digital sharpening techniques makes it much easier to control the negative effects of diffraction than film too.
It also matters which type of landscapes you shoot. 90% of my landscape photos are close distance shots often 4:5 portrait format rather than wide panorama shots with lots of tiny details at infinity. Actually the background is often slightly out of focus in my shots due to limits of depth of field (and my desire to get a subtle layering effect in the scene). The grand panorama type of shot gains more from having high resolving power as it's more likely to be printed big and invites the viewer to step close and look at the details.
I've attached one shot to show an example where very high resolving power doesn't really provide much value. It's made with a 33 MP back and you don't really gain more meaningful detail with more resolving power. I actually didn't have any recent shot which show the opposite. But the best example where you really want lots of resolution is huge environmental group portraits in panorama format where you have relatively small faces in the frame, the viewer is then very much invited to step close.
Personally I don't stitch any longer as I don't think it's a satisfying way to do photography. For the same reason I don't focus stack either. One scene in one shot, ideally without need to crop in post -- that's how I like to make photographs. It's all in my head though, and if you don't have any issues regarding photographic enjoyment when it comes to stitching it's a great way to gain that extra resolution when you need it, and I'd say in most cases when that extra resolving power actually means anything are in such situations where stitching and stacking works well. Of course stitching with 80MP back gives you more pixels faster than a 50MP back, but I think the importance of high MP count is in any case further reduced in this scenario.
