When a website has established itself comfortably among the higher positions of relevant SERPs, it logically would have done so because the webmaster overseeing the site pursued all of the proper methods of SEO to secure its prominence in the eyes of search engines' algorithms. Leaving such a website's content and structure static can easily cause that website to gradually suffer a reduction in its search rankings because Google's ongoing ranking processes take into account whether the site is making an effort to continually provide relevant new content. However, if a webmaster is reckless in their decisions over what to change and how to change it, they can just as easily cause a new problem that does a lot more to drive down their website's rankings than leaving the site alone would have caused otherwise.
In general, a website comes across to Google's crawlers as regularly updated and continuously relevant if it adds new pages through methods such as a regularly utilized blog feature. Alternatively, it can rephrase, rearrange, and expand the content of its existing pages while retaining the thematically important terms that were originally included in those pages' text and tags. Since those keywords are what Google uses to determine whether specific pages are relevant results for any given search query, a page on the website will suffer a massive drop in its current rankings if it undergoes changes that remove those keywords.
A redesign of the website's user interface should not result in Google's crawlers interpreting its pages to have "disappeared" so long as those pages remain located at the same URLs. Any change in the website's structure, however, will assuredly separate a given web page from its accumulated ranking credit because any URL change would technically mean it is no longer located at the URL the site would previously have been indexed at. This always happens when the website pursues a domain change, so the webmaster should always be prepared to create persistent 301 redirects that instruct Google to consider pages located at new URLs to be pages that have "moved" from their original URLs. For more information click here https://www.reddit.com/r/SEO/comments/bmewvc/whydoeschangingyourwebsitestructureor/.