If they decided to cash out for any amount, that would go against their track record of holding on to companies and exiting when they are much larger in evaluation.

This would not only raise eyebrows, but eventually the truth will come out. Put yourself in the shoes of a buyer, if you were buying out Sequoia's shares and after a few weeks/months news came out that they only sold their shares because of this. Not only that, but that Finix is pretty much competing with Stripe, which they have no chance of succeeding at, it would tarnish Sequoia's reputation and also open them up to litigation. As someone else in this thread pointed, $21 Million is a rounding error for them.