You definitely can’t do without improving website design when:
The site unexpectedly dropped in the SERP;
Google has introduced new algorithms and urgently needs to adapt to them;
A filter has been applied to the site;
The company has rebranded;
There have been functional changes in the company (the payment system has changed or the company has switched to a different audience);
There was a large number of site failures;
Found a lot of bugs in the code;