Expired memberships being erased by new memberships using Webform
Sounds like you want to check out how to send existing members a renewal link via CiviMail. Webform CiviCRM provides you a handy link on the CiviCRM tab and an explanation.

The key concept here is that you can send contacts a special/unique-to-them link to the renewal page that bakes in their existing contact id so that the page already knows them and their membership, making the renewal easier for them and for CiviCRM.