So did XP when it came out. You think IGPs are bad today, back when XP launched they couldn't even handle the rather simple skinning of XP's Luna adequately. Like with Vista, people who had actual video cards were fine, but everyone else was complaining about how slow XP was.
And, kind of feel like a broken record here, but development wise, XP was a dead end. Any developer will tell you that sooner or later a project goes off in a direction you never could have anticipated when you started and most times you're better off just scrapping the whole thing, or at least large chunks of it, rather than trying to shoehorn in something else. During the time XP was in active service we had the shift from IDE to SATA, PCI to AGP and then PCIe, hyperthreading, dual and even quad core CPUs, dual channel memory became a thing... That's just hardware. Software wise people started consuming more and more video content on their PCs, the explosion in processing power and cheap storage allowed games to become much more complex, battery savings on laptops and other portable devices became much more important necessitating shifting a lot of XP's internals from CPU to GPU, dual and quad core CPUs also represented a significant challenge for the Windows developers because there are a number of subtle but crucial differences between multi-core and multi-CPU which can lead to a significant amount of unnecessary overhead. Vista also migrated from the inefficient file based install to the much more efficient image based install, which is why upgrades from XP to anything but Vista require a clean install. File based installs worked well when you needed to break things up across multiple floppy drives, but doesn't really make nearly as much sense in an age of CD and DVD installs, which are now giving way to a purely digital distribution. There was some significant hardening of the security on the Vista driver model which was used with Windows 7 and even 8 and all the lessons learned by Microsoft over the security nightmare that was the first 2-3 years of XP's existence were applied to Vista, which, compared to XP, has led a relatively quiet security life. Same as its offspring Windows 7 and 8. There haven't been the roughly 1-3 remote execution vulnerabilities found per week, every week, for the first 2-3 years like there was with XP. It frees people up to complain about other issues.
When you look at all the thorny technical issues that had to be tackled with Vista, you come to realize it was practically a brand new OS written from scratch as opposed to just a continuation of XP's lineage. As big a disaster as you might think Vista was, it absolutely pales in comparison to what would have happened if they had tried continuing on with XP's code base.