In response to your question, "So what would the good version of this change look like and do you have a role in it?" - First, is that the right question to ask?
Maybe "this change" isn't the right one to make?
Perhaps a better question to start with is this, "What are Medium readers most unhappy with now, and how can we change that for the better?"
Next, "What are Medium readers, current and future, not aware could be done, but that would rock their world in a good way, and how do we accomplish that?"
The first of these two requires asking Medium readers and listening to them/us.
The second requires vision. It also requires a bit of humility (not saying you don't have it - I don't know you so can't say). Said diferrently, quoting a super-successful entrepreneur I admire, Danny Iny, having "strong beliefs, weakly held."
You need to have strong beliefs to have the confidence to try plausible things that you're not positive will work. Those beliefs must be weakly held, so when reality says, "Wrong!" you listen.
As for SMEs and credibility vs. writers without official credentials, why make it either/or?
Someone who's an expert in cosmology (like Dr. Ethan Siegel - @startswithabang or Prof. Avi Loeb - avi-loeb) can, should, and do say in their bios what their relevant background is.
That's why when it comes to astrophysics or the possibility of life beyond the Earth, I'll read what they have to say.
I'm far less likely to read Felicia Sullivan (@felsull) on those topics, but will definitely read what she has to say about marketing and branding. The thing is, I'd also read what Felicia writes about her life experiences because she's just an amazing writer.
My point with all this is that Medium should give us readers the credit that we can figure out which topics are ones where credibility due to official credentials is important for us, and which ones we're happy to just find someone who writes compelling articles that inform and/or entertain us.
Turning to personal finance, which is the niche I write about on Medium, I follow some writers I follow who are economists (like Ben Le Fort - @benjaminlefort) or financial planners (like Jake Northrup - @jake-61917 or Eric Roberge - @beyondfinances).
But I'm just as interested to read Jason Clenenden (@buildingarks) about many things financial, exactly because he's not an accredited CFP or CPA, but he's been building wealth through real estate investing for many years.
If you're still with me after all that, responding to your statement that you "didn't take the Medium CEO job to make bad changes," I urge you to consider what the best questions are to ask now, and what are the best answers to those.
And if you think credibility is important, perhaps expand the writer bio to let writers display their LinkedIn profile and/or any degrees and certificates, but then let Medium's readers decide if that credibility is important to them on the specific topic they want to read now.
Don't just throw out willy nilly anyone who can't prove they've been certified as experts.
