Convert ı to upper case and you get I. But convert I back to lower case and what should you get? i or ı?

This is a very strange question. In English you get i. In Turkish or Azeri you get ı. There is a very fundamental difference between ı and i and basically no native speaker mixes them up. When you're learning English, for example, you mix up and write capital İ instead of I. The rule is basic: ı <--> I and i <--> İ. No magic going on.

You don't always have the luxury of knowing the language of the text you're processing. You may well encounter both English and Turkish words in the same text stream, and you don't always have the means to distinguish between them.

It's the same problem with Han unification. It's fine if you have 100% Japanese, Chinese or Korean and set your font accordingly. But it all breaks down when you try to mix two or more languages in the same stream, unless you have some external means of distinguishing between them.

This might be true but Turkish alphabet was designed in late 1920s when computers barely existed. Maybe they could update it now? I find it hard to believe they would do it though.

EDIT: I guess you could make a case about Kazakh alphabet not inheriting this feature. But having written systems for both Turkish and English I don't find this a huge deal. I guess this alphabet reform has mostly nationalistic motivation and Kazakhs might have wanted to fit their alphabet to similar languages like Turkish or Azeri for which the alphabet is tested and we know it works.

> When you're learning English, for example, you mix up and write capital İ instead of I

That's part of the aforementioned fun.

Another part is bugs like this:

https://bugs.debian.org/579324

I mean, when you're learning a different language, isn't it normal to make mistakes? Why should other languages optimize for English learning?

By converting to uppercase you lose information. There's no magic way to avoid this, unless you store the original. Greek also has this problem with stress accents. For example, both "γέρος" (old man) and "γερός" (strong) become "ΓΕΡΟΣ" in upper case. Case round-tripping isn't available in all languages.

Lossiness is not the (whole) problem here. Bigger issue is the locale-dependence: in Turkish locale i->İ and ı->I, while in the rest of the world i->I. So it actually would be better if case conversation was lossy in this case. This is further excarberated by the fact that 'i' is in basic ASCII and widely used

We had a bit of fun with the mapping of the two versions of 'i' here: https://handsonturkish.com/typing-turkish-alphabet/