When basic SEO is left out of your site (for example, omitted after a site redesign), things go downhill very, very quickly. Even before Google reindexes your site, pages that used to exist now 404. Friction increases, traffic is lost; bounce rate increases. Within a couple of days, the poorly-optimised new pages take the place of the old pages.
Rankings plummet. Multiple listings that dominated page 1 of the SERPs become a single listing on page 8, page 20… or worse. Organic traffic, proven driver of the majority of your conversions and online sales, drops substantially. Sales suffer.
And all because someone (who ought to know better) forgot to do their SEO.