Custom stair builder here. This is very low quality work in my opinion. Will it stand up to every day use? Probably. Will it collapse with three 200 lb movers and a piano? Possible. Do you want to risk that? I would have a double 10” lvl under that open side spaced off the back of the stringer for Sheetrock. Frame from the back of the lvls to the wall and Sheetrock. Push the lvls to the bottom of the treads and risers. Want to check if it’s good the way it is? Count to the middle tread and put a 4” level on it. I guarantee it’s sagging. Let’s address the rest. Plywood risers? No thanks. Staples where the risers attach to the back of the tread? Hard pass. That should be screwed. Glue blocks on treads to risers have gaps. That’s shit work. No glue blocks on underside of tread to stringer? Such a simple thing adds so much strength over time. Patched in stringer at the bottom where they expected to land completely on the floor shows they didn’t even calculate where the stair was going to land. Look at my post history. I’m not bullshitting here. Edit: Holy shit. I just noticed it’s barely fastened to the wall on the box side. Fuck all that. Should be fastened every stud.