I think this is a Great Assessment Lily. And it rings very true to my experience in both the Poly and BDSM Communities.
The Point of Polyamory IS to have more than 1 Loving Intimate Relationship in ones Life, on an Open, Honest, level. Too many People either don't get, or ignore this factor. If there are secrets, lies, deceit, involved, it's Cheating. If it's Cheating, it's Not Polyamory!
Many who come into these Communities labeling themselves Poly, are seeking Sex Only Only Type connections. Which is also Not what Poly is about. And Which are desires that can be met in the Swinging Community. I think part of the problem, or mis-take in this, is that those labeling themselves Poly, are perhaps confused, or are a form of Community predator. And the label of Poly confuses the people they come into contact with.
If the pre-existing Relationship(s) are not Healthy, other Relationship(s) cannot be added with any reasonable expectation of success. One cannot (as you say) add another to fix what is broken or not working. Poly doesn't work that way, at all.
True Poly is an extension of Healthy, Happy, Loving, Open, Honest, Relationship(s). Not people looking for a bandaid so to speak.
One can put their thumb in a leaking hole in the dam, but how long will that last? Eventually it becomes a destructive flood that'll wipe out everything in it's path. My advice is > Don't be there!
There are (as you say) People who are Truly Polyamorus, who operate in their Relationship(s) with Honest, Open, Loving, intentions. One of the easiest ways to assess if this is what you have found (if you are seeking a Poly Style Relationship) is to be in direct Communication with all of the People involved in the pre-existing Relationship(s) Openly. And take your time getting to know them. As well as giving them all time to get to know you. If there are secrets and lies Anywhere, or vast differences in their communications to you, you haven't found what you are seeking.
In the BDSM Community there are many so called Poly Dominants seeking to add more Submissives, or Slaves. Assuming this is Their Right, and irregardless of the feelings, fears, thoughts, needs, or Agreements they have, with the ones they have. And without correcting that situation First. These do not make for a positive situation for those seeking a Loving Poly Home, or Relationship(s).
People in this Community are Learning and Growing more every day across the board in regards to what Poly is and what it isn't.
My experience in Poly is that people (like any other Style of Relationship) #1) need to Choose other(s) with the same or similar definitions of what they are each seeking for and in a Partner. Not just settle for whomever may be willing to connect with them. Keeping in mind people are Who and What they are. You cannot just take any person who becomes available, and hope to Change them later. #2) Work within the negotiated Bounderies and Agreements they already have to attain what they wish for in order to extend what they have built to someone else.
The Wise and Experienced Poly Dominants I know Never Choose other(s) that don't mesh with Their wishes to begin with. And They never add, or try to add, Anyone that won't mesh with what they have already established. These want and Will Have a Happy, Peacefull, continueum in Their HouseHolds, and in Their Lives. Something that cannot be attained without having done the work and being Responsible. This is easily seen in Their Homes, and Their other Relationship(s).
Respectfully~SidheWolf