Of course it is. It was a good OS and when offering the Windows 10 upgrade, Microsoft forgot about Vista and XP users.
So without a (free) upgrade path, I'm pretty sure Windows XP will linger on for quite some time. And even with a free upgrade, I'm not sure those users will want it, because most users are conservative.
Or these users are still rocking a single core cpu with piss poor gpus that just perform terrible on any new OS, even a modern linux distro. My sister still insists on using a laptop like this, so winxp it is...
It's not that simple for old machines. How about playing a video? Any new video player uses a 3D API for displaying/blitting. Guess what? This kind of machine has terrible OGL/DX performance and can barely play even the simplest of .avi. Winxp and it's typical software still has support for overlay video and whatever crap was used back then, so video performance is acceptable for non HD stuff.
What about Office? Wine is doable for older Office versions, but the performance hit is a lot more apparent on such pentium 4 era single core machine.
I've also found that lightweight distros are not enough in the cpu department. Their ram usage is great, you can easily find something that uses only 100MB after logon, which might be even less than winxp, but since it's all based on modern software you will quickly find out that even the simplest of tasks, like navigating with the file browser is a lot laggyer and more cpu intensive than that software for winxp that was designed for pentium 4 era cpus...
Sure you could go deeper with more barebones distros and/or only use the terminal, but then that machine is nearly useless for a common user, and may require a lot of setup and tweaking.
I think it is possible to do a mix. For example, there are several file-managers that are not heavy, I expect Thunar to work well on older machines, pcmanfm as well. For Office one could use something smaller like Abiword, though I never tested whether the performance of Libre Office is worse than Microsoft Office.
My experience with such older machines is really that when using a good software selection, it was a lot faster and more comfortable than Windows XP. However, you might be right about video. I have an Thinkpad R50 here and I was indeed not able to get gpu support for video playback from its Radeon 7500, with the cpu being too weak to handle even SD-videos. I was not aware that was a general problem XP solves better (this specific machine I never used with Windows) :/
GPU accelerated GUIs are one of the more dumb fads out there. Motif and FVWM run so much faster without a GPU than Mir/gnome or modern aqua or windows 10 could ever hope to even with a nice GPU.
So without a (free) upgrade path, I'm pretty sure Windows XP will linger on for quite some time. And even with a free upgrade, I'm not sure those users will want it, because most users are conservative.