Is ethical non-monogamy 'just a phase' for most people?
For some, yes; for others, no. The pattern looks like other relationship-orientation patterns: a subset of people experiment briefly and return to monogamy, a subset settle into long-term ENM practice, and a third subset move between configurations across a life. None of the patterns is exceptional.
Longitudinal data on ENM practitioners is still thin, but the qualitative pattern observed across communities is consistent: ENM as a life-orientation looks like a normal range of practising configurations, not a single arc from curiosity to abandonment.
Some practitioners do approach ENM as a brief experiment, find it doesn't fit, and return to monogamy with no regret — sometimes more clearly committed to monogamy because they have explicitly examined the alternative. Some try a configuration that didn't fit (couples opening too early, for example) and conclude the failure was the timing or the specifics rather than the orientation itself.
Others settle in. Polyamorous practitioners with decades of practice are common in established communities; long-term open marriages of 30 or 40 years are documented; relationship anarchists who have spent twenty years refusing the escalator are not unusual. ENM, like monogamy, has lifelong practitioners.
A third pattern — possibly the most common — is the move-between. People shift configurations across life stages: monogamous through child-raising years and openly polyamorous after the kids leave, polyamorous in twenties and monogamous after a particularly nesting-shaped partnership emerges, ambiamorous and shape-shifting throughout. The shifts don't index a 'failed' practice; they index a normal range of human relationship needs across decades.
What this means in practice: 'is it a phase' is the wrong frame. The better question is whether the person has found a structure that fits what they actually want at the life stage they are in, and whether they have the relational skill to sustain it. Both questions are answerable; neither is determined by the structure itself.