Traffic collapses after a redesign for predictable reasons: URLs changed without redirects, content got shortened for aesthetics, internal linking was rebuilt around a new menu, and nobody benchmarked anything before launch.
Benchmark before you design
Export current rankings, top landing pages by conversion, internal link structure and Core Web Vitals. You cannot diagnose a post-launch drop without a before picture.
Preserve URLs unless there is a real reason not to
A prettier URL is not a reason. If a URL must change, a one-to-one 301 redirect to the closest equivalent page is mandatory — never redirect everything to the homepage.
Do not let the new design shorten the content
The most common silent killer. Pages that ranked because they answered questions thoroughly get trimmed to fit a template. Design around the content that earns rankings, not the other way around.
Keep the internal linking map
Pages that had many internal links pointing to them should still have them. New navigation often orphans older, well-performing pages.
Launch sequence
Stage the site, crawl it, compare the crawl to your benchmark, test the redirect map line by line, then launch mid-week in the morning so a rollback is possible. Watch coverage, rankings and conversions daily for two weeks.
After launch
Expect a small, short dip while search engines re-crawl. What is not normal is a sustained drop past three weeks — that is a migration defect, and it is usually a redirect or a content depth problem you can still fix.