Adding cookie data should be a big deal because if everything in the cookie is important valid reasons for needing more data (or less data) imply major architectural changes.

For example, after a merger in which two company websites should be integrated you might start with one simple cookie, transition quickly to two independent cookies on two domains, then use a cookie with two keys when the two sites have separate backends but the same domain, then eventually a cookie with a single key again after extensive replacements and reimplementations.