For Google quality user experience is a great consideration in ranking a website.
Whether your site has a B2C or B2B focus, the website design and corresponding UX matters a lot.
Now, what do you think? Is the design made once and for all? Certainly not, the website design actually matures over time with several updates and upgrades. It is about perfecting things every now and then.
Your website can lose domain authority, page authority, trust, search rankings, and suffer terribly in slow business conversion if it gives an outdated look and feels.
Without perfecting things and tuning the design from time to time you cannot hold back what you have achieved so far.