Sexual incompatibility is rarely a good reason to open a relationship. In my experience, a relationship is sexually unfulfilling for one of two reasons -
1) The people have different sexual needs (this could be likes/dislikes/kinks/libido)
2) (At least) One person doesn't want to have sex with the other.
If number 1 is so much of a problem that you're thinking of shifting your entire relationship, it probably means you're just generally incompatible. Opening the relationship won't solve that.
I was meaning to comment on this, as Mags did (if you don't mind discussion on your "Not Really a Blog").
Similarly to Mags, I am in a long-term relationship where differing sexual needs are a huge part of why we are both poly/doing ENM. My partner has a much higher sex drive than me, more "complex" needs, more desire for kink, desire for lots of kinks I don't do, desire for more variety, etc. (And there are some sexual things I like that he doesn't really do, and although I don't have much motivation to seek out others, I enjoy knowing I have the freedom to do so).
Also like Mags, we were both already doing ENM when we met, so there was no "opening" a relationship. (In fact, kinda the opposite, as we fell in love from excessive casual sex, lol, and slowly fell into a serious relationship).
I get that you're only talking about opening a monogamous relationship in your points here, so starting out poly doesn't apply.
But it sounds like you're also saying that strongly differing sexual needs probably just means generally incompatibility for a relationship? I was pondering that because I don't think that's true...
But maybe you are talking about relationships in which the people are TOTALLY incompatible sexually, like not really wanting to have sex with each or having totally unsatisfying sex. Which, yes. Been there, and it was indeed an indicator of total lack of compatibility in general (but I was too young and inexperienced to recognize that at the time).
My current partner and I, despite differing sexual needs, are actually sexually compatible. I mean, we have really good sex and our kinks align with each other in general, so there is no PROBLEM between us sexually. And even though his sex drive is MUCH higher than mine, mine is far from low! So we don't have a basic sexual incompatibility (which is maybe what you are talking about in this post).
Anyway, I do think that ENM/poly is an excellent way to have relationships precisely because I believe that everyone's sexuality is complex, fluctuates over time, and no two people are 100% compatible sexually throughout their entire lives. (Or 100% compatible in other ways throughout their lives). A main reason I am non-monogamous is exactly because people's sexual needs will almost always differ!
But I agree that maybe that's not a good reason to OPEN a monogamous relationship.
But then--is there ANY good reason to open a monogamous relationship? Other than both people joyously wanting to embrace a philosophy of non-monogamy and both being willing to start over with a new relationship model?
Like, one person discovers they are asexual. Still seems like a complicated and painful way to enter polyamory. Or one person wants to explore their bisexuality. Well, we have seen how fraught with pitfalls that scenario is! Or one person falls in love with someone else and cheats emotionally if not physically--a terrible way to begin polyamory!
Couples that successfully open often already had a poly or non-monogamous mindset to begin with--like, they were comfortable with casual ENM in their 20s, or they were swingers who were surprisingly chill about falling in love, or they were already non-traditional in some other ways.
I don't know. I think it's super hard to transition from long-term mono to poly. Rarely seems like a good idea.