Because digital ignores the advantage of having a second resource to load data from.

Only if yu have 256 mb of system and GPU RAM to work with...

Disk is faster then optical, this is a non issue, that will be corrected. The retail  on 360 has the same issue so it has zero to do with a second source to load data from.

That 5.5 or maybe 7Gb at the end of the gen will run out too at some point. Plus reserving a large part of that for caching HDD assets would still be helped by having an extra resource to read data from.

Disk + optical is faster then Disk alone, if optimized correctly

the 360 retail version has only this issue if you want to install the second disc (for less noise as example) so that you only play from hdd but you can still just install the 8gb of one disc and use the other disc to play it exactly as on ps3 where you also have to install 8gb and play from disc. the only difference between the 360 and ps3 version is that you could install the whole retail game on 360 as option and that you don't have to switch the disc once when you start the game the first time on ps3.

not sure why you don't see why it is a big problem for the digital version in this case because this is the only version where you have to live with the problems. retail version for ps3 an 360 don't have this problem as long as you play from disc.

I see where you are coming from, but I think it is nothing more then a glitch that will be fixed as opposed to a real issue with digital.

http://www.eurogamer.net/articles/2013-09-16-rockstar-warns-dont-install-gta5-xbox-360-play-disc

"So what's the story here? Well, optimal streaming is achieved by making use of all the available bandwidth in the system. Why stream just from the hard drive when you can run in data simultaneously from both the disc and the HDD? Based on what we're seeing on the Xbox 360 version, perhaps running both DVD and HDD assets from just the one source slows down access times, impacting streaming performance. It's not game-breaking stuff, but it does take you out of the moment when it does manifest and for that reason we can't recommend installing the play disc."

Same issue as with digital.

Looking at the estimated pixel fill rate and processing power of next gen GPUs, I think its not very likely they will ever be able to occupy that much VRAM. You'd need a GTX 780/Titan like GPU to push 6GB worth of data.

Even if that day comes though, I'd much rather take no disk swapping, digital library following me, instant switching and other benefits digital will offer, over worrying about few games and their textures at the end of the next gen console cycle. 

I am also not touching GTAV till next gen version or PC version comes out... :)

Same, I'm waiting for a next gen GTAV version as well, too spoiled with 1080p nowadays :/

I don't think full installs are always better though.

HDDs fill up. They use constant angular velocity and work from the outside inwards, fast performence initially, gradually decreasing read speeds to less then half that of an empty disk as it fills up. Plus fragmentation (not neccesarily of the files themselves, but related files being far apart) will slow things down. DF already proved these effects earlier with GT5 load times on a heavily used PS3 vs a fresh system. I have the same thing on my desktop, disk gets full, my video editting software runs much much slower.

Developers have no influence over the state your HDD is in, or where the game files end up. They can however optimize the file system on the optical disk for optimal performance.

This gen had 512 MB of data to fill up with a 5400 rpm HDD and 9 MB/s optical drive
Next gen has 5-7 GB of data to fill up with a 5400 rpm HDD and 11 to 26 MB/s optical drive, why not use the drive.
Plus the 500GB stock drives for next gen will fill up fast, considering the current gen version of GTA5 is already 18GB.

You can ofcourse swap out the stock 5400 rpm drive for a faster one or better yet an SSD drive that doesn't suffer from degrading performance as it fills up. Or simply leave at least 50% disk space free.