Why not just migrate every single page from the old site?

I`m helping a new client with a site migration. Her web developer sent her this email: "There is a lot of links but to preserve SEO best way would be to go into search console, get data for last 90 days, order pages by clicks and copy any page with more than a few clicks - these are the pages you want to 301 redirect to the new site if any URLs have changed." I`m puzzled as to why we should not just migrate every single page from the old site? Even if some of those pages might not have had any clicks, they would have been part of the internal linking structure which helped some of the traffic pages get clicks in the first place? Does anyone here think they`re right in their recommendation? I feel they`re just trying to impress the client with some bullshit in order to win her business back. The next line of their email reads: "This is a huge job and usually part of our SEO package. Are you looking into doing SEO with us? If so we sort this out, if not I will need to quote you on us doing the following requirements as this not within any website design and development job."