Linguists tend to be a bit more specific than me. There may be a slight difference between /i/ and /j/, but they're really close. It doesn't seem to be enough to justify one being a vowel and the other being a consonant.

I tried listening to the recordings of /i/ vs /j/ on Wikipedia. /i/ just had /i/, but the recording for /j/ is /ja/, so it's hard to concentrate on the /j/. It sure sounds a lot like /ia/. Similarly, /w/ had /wa/, which sounds a heck of a lot like /ua/.

I feel like /y/ just means that you start out transitioning from /i/ to another vowel. You tend to emphasize the following vowel more. But since you could be transitioning to any vowel, it doesn't make sense that /y/ represents the transition itself. The only constant is it starts out as /i/.

A particularly interesting case is /jiː/ (Old English pronoun that is now spelled "ye"). It's clearly not just /i/, and /ii/ would sound identical. But it does seem to be somewhat of a palindrome. The /i/ at the end is extended longer, but the sounds are the same forwards and backwards. There's a slight change in the sound or emphasis between them, so it might be /ieiː/ or something where it moves to a subtly different vowel and back.