I hear you on the argument that it would be great to see genetic diversity moving freely back and forth. I think the issue is that most people who have put in a lot of time to build a collection would get run ragged if they gave scions or seeds to everyone who asked for them.
When I had a small collection I tried to make myself useful for collectors who needed help in exchange for plant material. I worked in trade for plants. I didn't have the money to buy all the plants I wanted but looked for way to source material and knowledge.
As I build my collection I looked for trading partners who had material I wanted. In fact I got some of my most rare varieties through trade with collectors around the country.
I slowly rolled my collection into a little nursery (I also have a full time job) and started selling plants on the side to fund my collection. I realize everyone can't do this because of space and time requirements, but it was my path.
Now I find myself in a different position where If I get a new species, it's not just a part of my collection, but also an asset. I've got cost sunk into getting the plant, likely some failures along the way, but at the end of the day I want to recoup the money I put into it and hopefully make some money to further my collection efforts
There are still quite a few plants I'm looking to collect(even with a nice collection to trade and willingness to pay), so you're not alone in wanting plants to more accessible, but as it stands, some stuff is just tough to find.
I think that when plants become more readily available people will be far more willing to share them, but as it stands, if I ask someone for a cutting that as a grafted tree would be worth hundreds of dollars, and I know they have limited budwood, I am essentially asking them to forgo their financial interest for the sake of my collection. My best work around for this was to literally pick up the scraps off the ground when people were grafting and I attempted to micro graft the pieces I could get. They were free but I had to be at the right place at the right time.
Sorry for the long post, I wanted to share some perspective and hopefully some suggestions to help you build your collection.
John