How redesigns actually go wrong
Nobody sets out to lose their traffic. It happens the same handful of ways, every time.
The URLs changed and nobody mapped them
The new system prefers a different URL pattern, so every page moves. The old addresses now return a 404. Every link anyone ever built to you, every bookmark, every ranking attached to those addresses: gone, silently. The site looks great. The phone stops ringing. It usually takes a couple of months for anyone to connect the two.
The content got trimmed because it looked long
A page with eight paragraphs answering real customer questions looks cluttered in a design mockup. It gets cut to two lines and a photo. The design improved. The reason the page ranked was those eight paragraphs, and it is now gone. This is the quietest failure of the lot, because the page still exists so nothing looks broken.
Nobody looked at the data first
Decisions about what to keep get made on taste rather than on which pages earn clicks. If you are not looking at Search Console before you start, you are choosing what to delete at random and calling it editorial judgement.
The staging site got indexed
The in-progress copy was left crawlable, and now Google has two versions of your site and no idea which is real. Alternatively the reverse: the blocking rule from staging shipped to production, and the new site is invisible. We have seen the second one sit unnoticed for weeks. Both are in the launch checklist because both are common.
Everyone walked away at launch
Launch is when you start watching, not when you stop. Redirect errors and crawl problems surface over the following weeks. If nobody is looking at the reports, the site degrades and the first signal is a bad quarter.