I disagree that all the endings are sad ones.
Yes, most are. That said, the
Aldecaldo ending is the happy one.
True, it does not unequivocally state "V is cured of their disease and lives happily ever after", but the hints are there - especially during the credits. Misty's Tarot reading (The Chariot, The Lovers and The Sun).
The Chariot: direction, control, willpower
The Lovers: partnerships, duality, union
The Sun: joy, success, celebration, positivity (also listed as Success, happiness, all will be well)
The game hints a LOT that the tarot are meant to mean more than just something Misty does to pass the time. The quest implies there's someone out there to help you make meaning out of life (i.e. the writers/devs).
So in the Aldecaldo ending, where you "fight on" as Johnny suggests, that's exactly what happens.
I also interpret the recordings during the credits to have occurred AFTER you were supposedly going to kick the bucket, especially Judy's if you romanced her as female V.
You may take it how you wish, of course, but that's my interpretation. Or in other words for the endings (in my personal definition of "happiest to saddest"
1.) Aldecaldo ending - You go back to world of the living, leave with the Nomads and your romantic partner (depending) at your side. With the resources of a full haul of tech and the unique insight of those you encounter in Arizona, you manage to beat the odds and live happily with your new, extended family (also keep in mind that, for all intents and purposes, V has no family prior to all this and all other endings finish without them having one as well.)
2.) Johnny ending - Any ending that ends with Johnny in your body. A happy ending, so to speak, for Johnny but not you.
3.) Go Big ending - The one that see V's initial desire to be "BIG" in Night City as their main draw. Makes sense that V would spend their days going down in an absurd heist in the stars if you stuck to that "Want to be immortal as a legend"
4.) Arasoka Ending - The "easy" ending as it requires no side quests to be done, which is very telling that (using RPG logic) that it is not MEANT to be a happy or "true" ending. This can end with you in Mikoshi (technically still alive as an engram, sort of) or going back home to live out your final 6 months.
5.) Suicide Ending - The absolute darkest ending.