I can see your point on that, although if thats the case it would be as easy to just drop another engine in, at £500 for a decent second hand unit. :-/
They appear surprisingly durable in kit cars, contrary to what you might expect. I tour with quite a few guys who run bike engines doing pretty high road mileages
and abuse them on track (you wouldn't put up with BEC "ergonomics" for road use alone!).
They have swapped the odd tired engine between them but IME they will last much longer than 10k miles. Most failures are due to the oil system not being properly engineered causing oil surge or scavenge pump drive failure rather than wear.
When you consider that you can drop a standard bike engine in from a scrapper (and bikers are always putting them through hedges) whereas at the same level of performance with a car engine you are probably doing at least a bit of tuning work, and hence investing time and money in the engine, I'm not sure, from a durability point of view, that there's much in it for a small, light car.
You'd have to ask how durable a Smart engine is against a normal car engine too, of course!
Kevin