Sounds more like pre-marital cohabitation (which is also already common), but perhaps with a more explicit and formalised sense that this is a time-limited 'trial phase'.

I wonder why this goes together with reduced expectations; I can imagine it leading to a resurgance in the belief that marriage is for life, by reducing the number of impulsive and semi-informed marriages. But maybe that goes hand in hand with the rejection of some less-realistic romantic ideals we have about the role of a life partner?