Are long landing pages necessary for a new SaaS launch?

First, take a look at these websites -

• https://ui.shadcn.com/
• https://vercel.com/

They’re very short and clearly explain what the product is about. However, in terms of SEO, these two are already extremely popular in their niche, so they don’t need to worry about SEO as much as new products do.

My question is - For a brand new SaaS launch, is a long landing page essential if it’s the only main page on my website? There are other pages, but they're generic - about, contact, terms, etc.