^^ The fact is that they need to port console versions to PC and not the other way tells how serious they are about PC gaming. Even GTA IV was a bad port even after delayed release.
Talking about optimizations, both XBOX 360 & PS3 have 512MB RAM compared to PCs with multiple gigs. I can understand what needs more optimization.
Plus they decided to push back the release a couple of weeks before the actual release date. The timing alone is sufficient to speculate that they haven't reached their XBOX One/PS4 sales target so the delay.
Many of your arguments don't have merit. It's true that Rockstar is not that serious about PC platform these days, but that's because making the PC version is much more harder. They can easily spend that time on other console games and its easier to milk the console users than PC due to relatively less chance for piracy. This is also the reason why console version prices are almost always higher because they can set any price they want to and most console users won't have a choice.
Secondly, PC is indeed harder target to optimize for. Writing a game for limited hardware is definitely challenging, but scaling it for higher end hardware is an even bigger challenge.
On console, you have only one target hardware spec per platform. You know that you have a 3 core CPU and 512MB etc. So, its easy to make hard calls like quality of textures to go with or rendering resolution and often even before you are well into full scale development. There is only one defined experience to target for all users of that particular console and that is what make it relatively easier.
On the PC side, you have users with dual core CPU, low end GPU and 512MB RAM and users with quad or octa core CPUs, 16 GB RAM and high end multi-gpu setups and there are thousands of combinations in between. A high end PC owner will not be OK with the same experience that a low end PC owner gets. A low end PC owner is not OK with unplayable frame rates because the game uses some hardware intensive features that cannot be turned off. Expectations will be high and optimization to meet the expectations of all sorts of users is a lot harder than with consoles.
I purchased GTA 4 on console as well as PC. When I tried playing GTA 4 on console, it was such a blurry mess that just looking at the screen gave me headaches. So I purchased it on PC as well when the PC version came out. It was considerably better on the eyes, but it was still a mess in other ways. So, despite having it on two platforms, It never got me hooked long enough for me to play it. I left it in the middle.
In any case, I have always been unimpressed with the GTA series thought it has defined the open world/sandbox games and has always been hyped, this series itself has never captivated me. There are massive number of open world games that I would rate higher than GTA. In fact, even from Rockstar stables, Bully and Red Dead Redemption are games that are so much ahead on my list that they blow away the GTA series into the nothingness of oblivion.
GTA is one series that has repeatedly proved to me that while graphics quality/rendering quality alone is the sole deciding factor for a good game, this factor alone is enough to break a game beyond playability. In any case, after GTA 4, I will never ever buy another GTA game on a console. I will buy it on PC or not at all. If Rockstar for whatever reason doesn't want to release them on PC, I am fine with that because there are tonnes of other good open world games to play.
I don't know how good GTA V is, but based on my past GTA experiences, If I have to choose, I would personally rather have a port of Red Dead Redemption on PC so I can play it another zillion times. I already own it on XBOX 360 and its the game I played the max time and most number of times on that console. I would rather have it ported to PC even if its exactly in its original form than get the new GTA game