Do you not know how to compute your Swap efficiency vs 1.9 efficiency?
Yes. And I have also tracked. I do much better using swaps for 4&5 than I would have done priming. So far as people who take those positions: some good some bad. Every world I'm in has at least some guild members who use the swap threads but not the 1.9 (and some who self prime until they get sniped). That's their prerogative, but it does mean that they can do better taking swaps that guarantee positions (even low ones) on my GBs compared with many of the other offerings in swap threads. Because not everyone uses the 1.9 threads, I also make sure that members who don't use them still have opportunities for high (BP) value spots. I often do leave 5th for cheap for players who can't afford the big swaps or 1.9x.
Note: you really can't effectively compute swap efficiency because it's highly dependent on the swaps you take. That's why I've actually tracked it. 1.9 works much better down to 3rd. 4th varies a lot (more profitable GBs like Arc and Inno are cheap to prime for 4th, most others aren't).
If you are willing to put down 100 fps on any Arc level 5th spot in a swap, you are nuts -- but bless you.
Again: it's a swap. I put 100 down, but I get those back on some other GB plus a small reward. It isn't a donation, and since I'm going to spend those 100 FPs to help level one of my GBs, I'm out nothing but I get a bit more back (yes, if you prime you get free FPs, but my point is that for the small # of rewards for 4th and 5th the free FPs end up being less than I get back from the swap).
If you're just swapping FP for fourth with another single player,
I'm talking about using swap threads, not direct swaps. I do use those but only for first and with cheaper high level GBs, and we track FPs over 1.9x so that we can be "fair" while working on different GBs at different levels.
And if someone is using swap threads for the lower spots after using 1.9x to fill the top 3, what's in it for the other members who are swapping with no chance for a top 3 with someone does on their GB? If you have 12 contributors, sounds likely that's happening. Unless everyone in the swap is already doing the same, being all at 1.9x, in which case I also ask why bother?
I'm not sure how to respond to this any better than I have above. You seem to be under the impression that every single player in this game is optimizing and that's very obviously not the case. Lots of players self donate way more than they need for 1.9x. Plenty more just dump on swap threads when they have FPs and don't really pay any attention to what they're getting back. I try to make sure that anyone picking up a swap for my GBs is either guaranteed a spot or at least has a chance for one (in the world where I don't have Arc80). In most worlds I don't really care. I collect and dump--sometimes on my GBs to prime for a 1.9 spot sometimes on threads to fill up others--and don't really care what I'm getting back because they're diamond mines and I've got plenty of FPs.