If people are invited to spend tens of hours a week in soaking in media, that time is being borrowed from somewhere and in the modern historical context that's the time being stolen from home care, crafts and extra labor, sports and physical activity, third place social time in the community, etc -- changes that can dramatically alter society if taken up by enough people and can impact personal wellness even just for one person.
Constant, widespread social media immersion is absolutely having an impact on social and personal wellness and we can actually look to those previously normalized shifts as our evidence that it does.
What we aren't justified do, even as some here try, is assume that this impact of social media immersion will be harmless or beneficial just because pulp novels and couch potato living haven't yet destroyed modern society.
As you note, innocuous/harmless doesn't seem to be where the evidence is pointing so far.
Social isolation starts with kids not being allowed to play unsupervised. Don't hear too many cries to increase unsupervised time we hear the opposite. A screen might be the one thing to help that.
That may not be always bad, mind you, but it seems pretty obvious to me that it has occurred and is occurring.
I could rant much more about it, but suffice to say that just because we don't think something is a problem anymore doesn't mean it wasn't (and isn't) a problem.