[AlexMuir]

Being lego, this actually solves a lot of my immediate worries:

Sets can be weighed back in. Missing pieces are held in stock and can easily be replaced.

Lego can go into a washing machine/steriliser.

It is light and virtually impossible to damage in transit (assuming they devise good packaging)

There is minimal depreciation, and demand for new sets won't be absurd. (vs textbooks and general toys)

You have to work hard to hurt yourself with faulty lego.

The only problem I see is (for my family at least) Lego was faddish. We didn't touch it for months, then couldn't get enough of it for three weeks. This is a difficulty for a subscription model.

Overall this is an excellent idea.