While most venture funds have a 10 year term, almost all are extended for at least an additional two years. The GP, together with the LP's, have lots of flexibility to extend beyond then if necessary, although there are typically negotiations around fee reductions and clarifying the path to liquidity. I have no insight into this particular fund, but it is very unlikely that it is purely fund term motivating Benchmark's desire to exit.

VC fund lifetimes put a hard time frame of <= 10 years on required liquidity, forcing founders (who have given up board seats to get those investments) to do things they wouldn't ordinarily do. It's a pretty perverse incentive, and I see the attraction of not taking outside money.

It is perfectly reasonable to distribute illiquid shares of a private company at the end of the fund lifetime.

What is the lifetime of the fund? Just the amount of time that the VC firm has said they would have all the money (and more!) back to the limiteds? Clearly that's not a hard deadline, right?

It is a hard deadline because it is in the deal documents when the fund is formed. It is relevant because the fund management is guaranteed fees for that period. Extending it is usually straightforward, but the compensation of the fund management team during the extension is open for negotiation.