That's a double edged sword. If Boeing begins the design of a new plane now, they would be planning on finding customers in 10 years who expect to use the plan

That's a double edged sword. If Boeing begins the design of a new plane now, they would be planning on finding customers in 10 years who expect to use the planes for the following 20. But in 5 years we should have a good sense of whether Starship is viable. If it is, then 10 years from now Boeing won't have a market. Even if you give Elon only a modest chance of success, it makes sense to stretch out the sales cycle for existing planes and delay starting new development for 5 years.