Romantic perhaps... but impractical. It may be the last
Victorian sales gimmick to die, this model [if memory serves, there's such a gradual pearl delivery in 'The Sign of Four' - one of the Sherlock Holmes novels]
Not sure what the romance is worth to everyone, but consider the fast changing world of pearls: what if one had stared a strand like this of fine freshwater pearls a couple of years ago... with s much more and better available today, they may abandon the measly pearls that were the pinnacle of quality when they started.
With Akoya, the add-on strand might be quite missmatched (if the older pearls had started to break down) and rather long (with dropping prices, one may add more than just a pearl per year, LOL!). The other 'South Sea' pearls may reveal an interesting trend-line too, if one were to shop thoroughly for a specific thing each year, particularly if the item is of a scarcer kind...
On the other hand, multiple-year projects (that aren't school, mortgages or career changes... let's say) seem to be getting scarcer these days. So.... just maybe... there's sense still in the 'add a pearl' idea. Even if cultured pearls are no longer 'rare', a thoughtful collection might mean something. That's commitment!
A strand of matched pearls makes little sense for this, methinks. But with strands of unmatched pearls (multi-colours at the very least) ever more popular, I'm starting to think that the idea might not be dead just yet. Besides, 'partial' strands (three, five pearls, seven, etc.) are also 'in' (on chain, mostly, but...).
Besides, a large undrilled pearl of surprising color and nice quality under the tree might just start one on pearls for good. Then... they might just add a strand themselves!
One may never just buy a loose pearl - especially one of better quality, just to play with it awaiting a strand or a setting. And that's one quality of a desirable gift item right there (something one may want but are unlikely to buy). Perhaps not the most practical gift though...
Not sure about getting into a pre-made sales scheme for this though. Not at all
If anything,
I'd start with what could become the centerpiece of a nice saltwater strand in order to provide good motivation for the followup
Just a thought