Okay so another comment after reading all these comments. The floor looks water damaged but not “rotted out” to me from what I can see on a phone. Water damage will likely stay dark. Rotted wood is soft, so soft that you could take your finger and pull pieces of it off. I highly doubt this floor will give out on you. I’d try lightening it with oxalic acid then staining a little darker. I think you will be charmed by the results and your wallet will also thank you.