Both Brown and Havenstein will be UFA after this year. So will Saffold, who will be 31 going into 2019. Combine that with Whitworth and Sullivan- and it's reasonable to think one or maybe both could retire after 2018- and the Rams are looking at probably replacing 3 starting OL next year. Possibly 4. I do think they'll keep one of the three UFA, either extending Saffold a year or two or re-upping Brown or Havenstein (I would actually lean toward Brown over Havenstein myself. I thought he came on strong as the season went on last year).
So yes, the immediate need is OL 'depth'. But looking in the not-very-distant future the OL will undergo major changes next year. It's in their best interests to get a jump start on that in this Draft.