I think this is more than a little unfair on Upwork.
I've been copyediting and proofreading on Upwork since 2020 and have found it to be a really efficient and effective middle man. It's an agency, so of course it takes a cut. 10% in my view is a very reasonable agency fee.
I don't mind that it courts companies and big clients in an effort to persuade them to use its service. It doesn't need to court freelancers. But freelancers do need to find a range of lucrative work on its platform.
You fail to mention one of the biggest advantages of Upwork — the escrow payment system. The client has to pay for the job upfront before the freelancer starts work, so that when the job is done there should be no issue regarding prompt payment. I've not had a single issue with being paid very quickly once a contract has been completed.
I like the fact that ALL my freelance work is undertaken via Upwork. It makes end of year tax declarations so simple :)