We are somewhat beyond my ability to make an accurate statement here.
And I think to a certain extent, I'd reword the question as "how likely is it that Reynolds will take the step the team wants from one of its top 2 receivers compared to Johnson" because, let's not beat around the bush, what Johnson was doing was not a whole lot. And yeah, there's a bunch of reasons for that beyond his control, not trying to slag him off, but Reynolds can replace what Johnson was doing in his sleep. He had the same number of TDs and only 109 less yards despite getting 23 less targets.
But the Steelers' WRs need to be doing more.
And I suspect it's not super likely. I doubt he's suddenly going to blossom into one of the league's better WRs at 28. He should be solid, and I can see him posting good numbers if the rest of Smith's offence gets rolling and he gets a lot of targets, but I think signing him should be accompanied by drafting a guy the team expects big things from. Maybe not a first rounder, but they shouldn't be leaving it later than the third unless this draft is absolutely stacked (which maybe it is tbf).
But I think he's the best guy out there in FA.