I must disagree. Do we (or most of us) actually weigh the benefits vs. the disadvantages of the long term monogamous relationship or do we simply follow, like sheep, the dictates of our culture which, until recently, said one man, one woman...for life?
In our younger years, as teenagers for example, we do follow (or at least a good portion of us do). Although teenage is the time to test values and boundaries, this one is one of the most deeply ingrained since childhood ("they lived happy ever after"....). However, after trying a couple of relationships, most of us smarten up. I am sure that by our early 20s, when we accept or seek a new monogamous relationship, we don't get into it naively or blindly. There is a reason for it: fear of being lonely, desire to "fit" in a group, desire to have a family, or simply fun to be with her: whatever it is for each one of us, there is a reason, something against which we trade our independence.
And yes, good sex is part of it. If you didn't get good sex at the beginning of a relationship then she must have brought you something else... You must have traded your independence against something. But my point is that things evolve and the reasons that were present initially do fade. And good sex is usually one of them....
Arrangements occupies a niche market, between total independence and total committed monogamy. Let's be real here, there are many gold diggers pretending to want an arrangement but who are simply looking to scam or trying to get maximum gain with minimum involvement. I've had girls asking me, with incredible arrogance, $10,000/month, telling me they're worth at least that! But those girls who are truly looking for an arrangement and are happy in long term arrangement do it for a reason similar to ours. And when you meet those girls, there is no bitterness or little mind game, it's a fair deal.
I know my gal is very happy about our deal, as I believe yours is, and I see no reason in the short to medium term that will change that. She gets much more than financial help, she gets access to mentoring and my professional network that girls of her age don't have and that will help her tremendously in the long run. And yes she does have daddy issues and finds in me someone who cares for her and value her instead of someone who beats her and despise her. On my side, I do get much more than just good sex. Good times, tenderness and affection is something that cannot be purchased. But we both know it's a deal. Affection and even some form of love (or emotional attachment, call it what you want) is present, but no possession. However, at some point the balance may tilt, maybe some of her needs/motivations will change, maybe I'll get bored and want either more variety or stability, who knows? I see these evolving just as stable monogamous relationships do.
Now, as some have pointed out, sometimes it evolves into something that both partners still find worthwhile. And that's quite fine, good for them, no judgement from my part. But it is far from being the universal model that society would like us to believe.