As a big fan of rewrites (but not as big a fan as some), I think Linux has avoided needing big rewrites with a lot of very clever modularity in its design. Many corporate software programs do not have that level of modularity. Modularity has performance costs, is slower to develop, and "looks ugly" on a block diagram, but it pays dividends for open source projects and projects that need upgrades over time.

Individual parts of Linux go through relatively frequent rewrites (eg filesystems), but not the overall framework.