Smith is ranked #11 and Free #16 in the league out of 78 tackles. Free was the top tackle for several weeks. There is no need to spend resources on what is not a problem. They also like Parnell and Weems. JMO.
They changed the OLine scheme this season to help cover up Free's issues.
They have the RG keep a closer than normal spacing between himself and Free. To compensate they have the Center help block primarily to the right. The LG rarely gets help from the Center despite being a 1st time starter that had zero experience in real NFL games prior to the beginning of the season. This change minimizes the outside pass rushers ability to beat Free to the inside which allows Free to over-set to the outside giving him an advantage on the rusher.
They also alter the way they call running plays because of Free. They run just inside or just outside on the Left behind Smith; however, they try to avoid doing it on the right. The runs to the right are usually between the Center and RG or wide outside. When they tried going behind Free against the Raiders, he was called for holding and Murray's TD was nullified.
Free might look good in the PFF ratings; however, in reality, when you see Leary get beat at LG, it is often because the entire scheme is compensating for Free instead of giving help to Leary. On most OLines, the 1st year starter (Leary) would be the guy getting help, not the veteran player with the big contract.
I agree that they have options at OT and have a bigger need at Guard. They basically have no backup Guards on the roster. Costa is the only interior backup and he is primarily a Center. They have had Weems practice at Guard but I don't know to what extent. I would like to see Parnell challenge Free in training camp next season. Unfortunately Parnell was injured for most of the pre-season/training-camp and didn't have the opportunity to unseat Free this season.
IMO, an injury to a Guard would be a bigger problem that an injury to a DLineman. It would be unwise to go into the off-season expecting Waters to return next season. He is a nice option if they have injury issues in training camp, but otherwise, they need to add a real Guard in the draft or Free Agency.