I'm not sure how to make the decision about how to finish my 100 year old fir floor. The boards are 7/8" thick, but quite splintery. They have stains on them. Even after sanding 1-2mm of material off, they still look kind of dirty.
To tell the truth I rarely find old growth wood too far gone to save. In some cases I have had to replace rotten wood with wood from other areas of the house that were not as important. Taking a few boards from an attic or storage room to replace rot from a leaking window or chimney and returning the floor to its former beauty is really rewarding. You have some dark spots, but they can be bleached. Nails can be set.
The great thing about these old floors is that even after being refinished 50 years ago and 25 years ago and now ready to do it again you have thicker wood and 5x the quality--look at that tight grain. It is awesome. Do you see any knots other than bird’s eyes? I doubt it.
No more heavy grit; go finer. Read up on bleaching those small spots, then you are ready for whatever type of finish. Danish oil or stain and polyurethane are the two that I have used most often. I am partial to polyurethane as it brings out the beauty and can last decades protecting the wood from damage.
So no, I don’t think that floor is in bad shape at all.