Emails disappear from Mail Inbox

My wife like to keep all her Emails in the Inbox (except for deleted ones). I have tried to get her to move any she wants to keep into a mailbox, but she wants them in the Inbox, and on an older laptop there are several thousand in the Inbox. Anyway that's what she wants, but her newer MacBook Pro is removing Emails over maybe one week old.

Is there anyway to force Mail to leave Email in the Inbox unless we want to individually remove them?

Is this a result of installing latest OS 10.13.6 High Sierra?