Its nice to see how each of us has a different perception on what a Song is to them. There are no Rights or Wrongs in here.
I ain't a musician by professions either (Software/tech Consultant). So Most of the music that I do is for fun and Sanity
I mostly play improvisational music etc, but at times do record songs or pieces.
For me the song usually is based off the current state of things,mind, mood or the current feelings about ?, So changing that later in future is usually no go for me, as I am not in that state anymore, so any change I do will only take away from what that song is reminiscent of , like a snapshot of that moment in time.
Yet there are songs/music that I have done purely for non-self purposes (jingles/demos/presentation /for friends or other commercials) those unless already consumed by the use case , always have or may have the scope of improvement so keeping options open has always been a good idea.
on an another analogy, consider your-self an artist, say a painter , or a sculptor or any else, the most important thing here is the Art-Piece itself, not the tools you used,or the paint you used or the brushes etc. Yes they impart their own character to a certain degree, yet you being the sole creator do have the say in what makes that art piece what it is, That's your expression, you get to decide when it's final or done (though the hardest part). Thing to note here is You and Your Art, with times to come you'll learn the new tools and will be able to express yourself with them as you have been able to learn and express yourself with current generation of tools. Software or Hardware should not matter today or tomorrow, but what you do with that.
As has been advised earlier, Saving Stems has always been the choice of professionals because that removed a lot of the intricate dependencies.
A lot of this has to do with the use of multi track recorders and how that helped shape the things,
If for a moment you forget you have midi recording, then what all are you left with: Just Audio. Use and Keep that and you'd be future proof for most of the things. You could replace synth bass with a real player or guitars etc, yet keeping everything else intact, need different overheads, perhaps a different kick etc,.
You could have different mixes, or Master Setups, those who have been recording for some time would immediately realize how these hings have changed over time to time and come back etc (especially the Dynamic Range).
With the Advent of auto tune systems like Melodyne etc, you can even edit your instruments audio take and correct timing and pitch of played notes if need be.
All of these use cases and many more require that you at the least have stems (different audio track for each instrument). You can save some work if you store Midi but that is not future proof because although some things are standard like note numbers , a midi will contain automation cc parameters or worse SYSEX. If you play that midi with any other synth engine you are more than likely bound to find it troublesome unless the new synth also uses exact same parameters and cc mappings (very unlikely).
The Midi at best can serve you as a reference unless it's utterly simple (but if it's simple enough you should have no problems recording that again in the first place) , so do remember that.
The above has been my experience over the years (maybe it can be helpful to someone),
If I write anything on Kronos Seq as a Song that I might need to revisit, I save the AUDIO Stems in my PC). I stopped saving my final projects as midi, as that served me no purpose when the need arose. Though I do Use Midi extensively while working on the Project.
For me it would be counterproductive to worry about the tools, of today as I don't know what the future holds for me or my tool, the Only constant that matters to me is my RAW references (Tracks/Stems, that I Know will be the same, with the least dependencies).

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