Yea, this. An app like this is a total and legal potentially huge competitor to Lego. I’d wager Lego will happily pay 100 M€+ if this would never come to fruition in order to keep the new box with book flow not slowing due to this idea. The cat is out of the box however. This is disruption in optima forma.
Since downvotes are not my thing, I’ll explain my logic. The value of Legos is return customers. Build, break, drop it in the pile and never build again. (This with great fun obviously, but my experience is no box stays separate in the long run.) Any outside disruption that massively increases the replay value of boxes (perhaps even find that old box from among 20k pieces?) is a threat to Lego. They make about 1 € billion a year on 6 € billion gross. You have to take the lifetime value of the disruption, hence my 100 M€ estimate. It is an estimate that says the repeat value of Lego will rise for the customer with a fall in repeat sales for Lego in the 1% range.
I tend to disagree. Lego's main competitor isn't their own old sets; it's alternate entertainment like XBox or YouTube.
I would estimate that any innovation like this that encourages people to pull out and play with their Lego strongly increases the chances that they'll sell new sets to those people.