You realise all keys are completely transposable? Major being different to minor is not to do with sound qualities it's just that different intervals are used. If you're recording live instruments then the key would affect the timbre but production-wise it fuck all.
You are right about everything being transposable of course. And I am aware of what the difference is between major and minor. But if you transposed a song from A flat minor to A minor it would sound different. The relationship of the notes stay the same but different notes are used. A is not the same note as A flat.
When you say that
All the 3 or 4 not tunes about could fit into several different keys because they share the same notes, it makes no difference to anything.
I'm not 100% sure what you mean, but I think you're refering to the fact that 2 keys could have the same notes. (C major & A minor for example) The difference is what the tonic is and how the notes are functioning.
All I'm saying that if your writing a song, using different keys is a way to spice things up. If everybody just wrote in C, things would be pretty boring.
The question I have for you is, if all keys are the same then why do we have them at all?