B2B would be an obvious exception. If a term has real meaning that everyone is familiar with then of course it will have value. But just a random combination has no value in today's market.
The problem is 5YC only means Five Young Canibals to you. No one else would have a clue what you are thinking!
Not only are letter-number combos doing poorly, pure 3-letter .nets and .orgs have been going very cheap lately. I bought several 3-letter .orgs for $12.50 each recently and I know one big deal for pure 3-letter .nets that went for even less. When you can get real 3-letter .orgs and .nets for that kind of money there is absolutely no reason to be wasting money on number-letter combos that are a dime a dozen.