There really is no need to replace Gui and Stephanie, they don't do that many events now, they do a lot with H&MT than would just go on without them.
Maybe this was always part of their plan, get to know Lux well, do some duties then retire to another country to start a family.
That's seems a very unfair assessment of the work Guillaume and Stephanie do. Over the last 12 months I count only 12 events all four of them attended - and I would expect several of these would be events that Guillaume en Stephanie will try to attend when living in London as well - such as New Year, the Octave and National Day; they might skip the garden parties or some audiences. There were 4 more events that either Henri + Guillaume (2), Maria Teresa + Stephanie (1) or MT + Guillaume (1) attended that could indeed go on without them rather easily.
However, I am not sure how they are going to replace the 8 foreign trips Guillaume (and Stephanie) made last year. Nor the 46 solo and joint engagements in Luxembourg.
The only family member that is more active than Guillaume is his father; his mother has fewer engagements and is probably on par with Stephanie.
The only anomaly I found was that they didn't have any engagements in the month of April, so either they took a long vacation or had other pressing issues to attend to.
Note: the official engagement count will be higher as I counted multiple engagements related to the same event only once. As especially foreign tours include many engagements, the current count of about 75% engagements by G, S or G+S and only 25% with H and/or MT is probably conservative.
P.S. It might have been the plan to attend this particular college for quite some time but I don't believe for a second that they have been purposefully postponing having children. They themselves said they were hoping for a large family, not starting immediately, but within 3 years (and as that is what they said publicly, I assume that was a rather generous timeline as they most likely didn't want people to start asking questions too soon). It's getting close to 6 years now...