I've run into this discussion or question a lot on Android forums. Droids, of course, are much more friendly towards modifications than Apple products, yet I have never seen a useful Linux installation on a Droid. Driver problems are nearly ubiquitous, especially wi-fi, touchscreen and display. Even when an actual working desktop is achieved, the systems I've seen and heard about have run so slow as to be worthless. In every case I know about, including some of my own, a custom Android ROM has been far superior to a Linux installation, or even a stock OEM ROM. As far as attempting this on an iPad, I think you'd by stymied at the first step: finding a kernel and bootloader for it. Unless you happen to have access to Apple's software archives, and can get copies away without getting caught, you're out of luck.
There are a few dozen of us (well, maybe a few million) with iPads 1, 2, etc., which are no longer supported by Apple. I have two Windows 10 desktops, a Linux Mint desktop, a Windows 10 notebook, two Amazon Fire 8 tablets, and an iPad 2. So I'm blessed with productive machines (except for the iPad). My main desktop is a fairly new Dell, and my Amazon tables are also new – the remaining platforms are older. I rarely use the iPad 2, and envision it as being a great platform for an Open Systems OS. I perceive two big catches: (1) I'm no Linux software engineer, and modifying Mint for the iPad 2 is way beyond my aspirations. (2) Linux seems to be keyboard-centric, and unfriendly to touch screens. Is there some version of Linux or even some other OS in my iPad 2's future? Has anyone attempted or successfully done such a thing to their iPad? Looking forward to your answers.
--Submitted by Bob M.