Good luck trying to get Sequoia to accept common shares from employees in exchange for diluting their own capital, when they already get preferred shares in a different class through their investments in individual companies.
Sure common stock has a discount to preferred stock, but it's not an infinite discount, and it's mostly applicable in the pessimistic scenario. If employees are exercising their options, it's usually because the portfolio company in question has performed well.
Sequoia would most likely be happy to acquire shares in those companies at reasonable discounts to preferred, which would make both parties happy. With a permanent capital structure, the most logical thing to do is for them to keep levering up positions in their winning positions.