That's like, a great motivation to assign said senior AWAY from coding! "Single points of failures" are your greatest enemy, your senior my get a better offer. Or might get sick. Or might get in a car accident. Why would you want to bet your entire company on a single person? I mean... sometimes you have to do it because there's no other way, but it's a big warning sign, and you should diligently work to get out of that situation ASAP. "Pushing deadlines" might very well be a price worth paying in a situation like that.