I also don't think it's possible to wear out a cassette in 400 miles, even if you were actively trying to do so. Cassette looks fine to me. And the real thing here is, if the cassette were indeed worn, the chain would be absolutely shot. If the bike shop didn't recommend replacing both, they're just trying to make a quick sale for no reason. Maybe they have a take-off 5800 cassette in the back that they'll try to put on your bike for retail.
Beyond all that, when I pull a cassette because it's noticeably worn (that is, shifting has begun to suffer,) the 11, 12, and 13 are usually in pretty good shape, because they don't get used nearly as often as the 5 cogs in the middle.
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