I think they serve their purpose fine. I know there might not be happy endings, but there are definitely brighter ones:
Bad endings: Everything you unlock through main story missions, so Arasaka and suicide
'Bad' endings: Almost all that unlock through Rouge and Panam: V in cyberspace, Johnny takeover & bad secret ending
Neutral/Good endings: Good secret ending & leaving with the Aldecaldos
That's how I'd rate them, roughly, in my mind.
The Aldecaldos route is basically Arasaka, but better for V in almost every regard: You have 6 months regardless, but you don't waste weeks in orbit, keep your relationship, don't cause an Arasaka dystopia (Yorinobu takes over, kills Hanako) and if you choose to become an engram you are merged with Alt behind the Blackwall - "free" - not stored away, without rights and at the mercy of Arasaka. Shame Smasher does his thing with Saul's head though.
Secret ending is neutral/good because that's the true 'Become a NC legend' ending.
Yes you're probably dead in 6 months, even in the 'good' endings, but it is somewhat left to player interpretation in those. Is it likely there is some kind of cure for V? No, but those endings also don't show V in a wheelchair, brain deteriorating away - we're shown V driving into the sunset in one and V going to start a legendary heist in the other.