Are you sure those are oak treads? Looks like they could be southern yellow pine to me..........

Southern Yellow Pine is perhaps the most popular wood for stair treads (pretty close with oak). Predominatntly used in basement stair, painted or carpeted stairs. But it is definitely considered a "hard" wood and stable enough for stair treads.

i would use keep the pine stairs. use a dark stain and add a runner to both protect and dress up the stairs.

I would use the new treads. Probably best bet is to go a couple of shades darker than the pine, so that you get a distinct contrast rather than a near mismatch.

I just did my floors...red oak, white oak and pine (old house mix). I used aniline dye from w d lockwood. My floor guys had never used it, but since we did samples and their stuff couldn't show the grain like the dye, they gave it a go. We are all very happy. Did a wax oil over it for protection. My current floors (we are moving), I did the same dye but with tung oil. 10 years and they have worn great. And I have dogs and don't take very good care of the floors!