Last few seasons, SHIELD is just trying to survive, not really going on the offensive against bad guys.
One or more of them is always in danger of being separated from the other main cast members, by time, space, reality vs. virtual reality, etc.
That has become the norm, that this team will risk everything to get everyone back. So the season-long arcs have been about getting everyone back.
What the hell are they going to do if they ever get everyone back for a year or two? Great things? Just fight for whatever crumbs the Marvel movies leave for them?
For whatever reasons, Disney just wants to keep this show on the air. So viewers get photogenic cast members and some bargain CGI.
There are some theories, like the show is really popular in China or something like that.
Maybe Disney figures the Marvel Universe is so lucrative, they can carry at least one TV show. Once Disney+ is online, I'm sure they'll do more. And I was going to say if Agents of SHIELD is ever canceled, it could go to streaming. But Disney owns ABC so they get to decide that.