>>If someone like Donald Knuth would be forced due to economic circumstances to join a trendy web start-up, he'd be a junior working under a twenty-something senior with little more than some Javascript and MongoDB knowledge

To play devils advocate, why wouldn't Donald Knuth be the junior given that scenario? In that position he may actually know less relevant knowledge than the junior. I mean many startup/crud apps don't require advance algorithmic knowledge, more front end dev and ability to deliver relevant business value. I wouldn't expect that junior to join knuths team and be put in charge, why should we expect the reverse? Given time yes, If Knuth is more intelligent he could become better at startup relevant work, then he should be more "senior" not before.