I suppose the best reply to this is that for a healthy romantic/sexual relationship to exist the partners in it must already be basically emotionally healthy, including having the coping skills and support to deal with their own emotional disregulation if it exists.
It's wonderful to see happy partners in lifelong relationships. That is almost certainly a sign of individual emotional maturity and self-regulation. Those people would be happy in or out of that particular relationship (absolutely not discounting the intense grief of losing a lifelong partner, but the emotional resilience to start enjoying life again), and the ones you mention clearly have strong and healthy relationships with other people as well.
Unhappiness with life before finding a partner demonstrates that there is something fundamentally wrong. People feel too lonely, or too insignificant, or too unloved, or too undervalued, or some other excess of negative emotion that at its root is unhealthy self image or mental health issue and needs to be dealt with independently of whether or not that person is in a partnership. Validation from a relationship can certainly mask the underlying negative emotion, but there's a big risk that at some point a partner's validation will stop working and the person will become unhappy again but put the blame on the partner or the details of the relationship without realizing that it's the same internal emotional problem that was always there. Almost certainly if the relationship ends the person will think the partner left because of the incorrect belief they have about themselves due to negative emotions, e.g. "I was unlovable" or "I wasn't attractive enough".
> There is a bizarre (and imo destructive, and toxic) idea that seems to be running through tech especially that devotion to your partner is rooted in "jealousy". It usually leads to "maybe you guys should become polyamorous. What are you jealous?"
That is indeed toxic. Jealousy is a natural feeling; it can be rooted in some negative beliefs but not always. To me it feels like a fair mix of instinctual response from relationship preference and underlying fears. Unfortunately for the folks you mention instinctual jealousy is a pretty strong indication of a preference for monogamy and not for polyamory. In contrast what I've heard called "compersion" is a feeling of shared joy and happiness about a polyamorous partner's experiences with other partners, distinct from any sort of fetishization of another relationship (which while not necessarily unhealthy is distinct from simple happiness at a partner's happiness). Jealousy for me has usually been rooted in fear of loss; loss of a relationship or fear of missing out. For monogamous people jealousy is also rooted in, for lack of a better word... Monogamy. It's probably the majority relationship preference.