For some organsiations - specifically two that I work for, gradual enhancement is the practical reality of managing their websites, while a complete rebuild might be a long term vision.
Take organisation number 1. Huge site, vast number of users across various segments, millions of visitors each month. Given the complexity of the business, internal and external stakeholders, pressures, egos etc gradual enhancement is the only way to get things done. BUT -- its on an overall roadmap. There is a "grand plan" albeit at a high level, that helps prioritise work, and specific changes are usually tied to work else where in the organisation. Its the only way to get funding.
Now take organisation number 2. Vast number of sites - 59 in total. No consistency in segmentation, and traffic varies from site to site. They are desperate to consolidate, rationalise, simplify and create consistency, not just from a UI/UX perspective, but infrastructure and content management aspects as well. They don't have the budget, skills, technical capabilities, design resources etc to deal with a large rebuild programme. Even tactical changes take a while to implement. The only way to keep any improvements happening is to take small incremental steps and a gradual release of new designs and functionality.