What you said there in your first 2 paragraphs is what I always said about people in these situations, because I genuinely thought that people would leave and find someone who wanted to have sex with them, and I always thought the one who didn't want to have sex should be completely honest so their partner knew what the situation was.
However the more I read about this elsewhere, I saw that that's not the case at all, and a huge amount of people in sexless relationships have "the talk" over and over or try to, even when it's absolutely crystal clear that their partner doesn't want to have sex with them.
Often it gets to the point where the other person won't speak about it and it's fairly obvious why that happens once you read the stories, there were many arguments or discussions about it, their reasons are considered to be 'excuses' and not accepted. Reasons such as "I have no libido since I had a baby" or "I'm exhausted". People complain their partner kept coming up with new excuses, it's clear to see that that was most likely because their other reasons weren't accepted.
Add in the fact that many people go through with some sex they don't want at some point to try to get their partner off them back/keep them happy and there's a real recipe for disaster because many develop an aversion, and most people don't even understand that they have an aversion so they don't have the words to describe their experience, but are made to feel like the bad guy and that they're doing something awful.
In the OPs case, she said sex was always sparse and infrequent from the very start, she always had a low libido. It's not zero, it's still just sparse and infrequent. She said in the early years he said he'd like it more but now he doesn't comment on it. Perhaps he's one of the rare people who seem to have a healthy view on this, he'd like more, but doesn't want more from someone who doesn't want it, that's really the only healthy attitude to have if you choose to stay in a relationship like this.
I have a high libido, I wouldn't get into a relationship with a man who only wanted sex very infrequently, and if I did then it's not on him to keep checking if I'm alright with it or to 'let me go'. He wouldn't be doing something wrong to me, the onus is not on him to free me from a situation I chose to be in.
You said if you didn't want DC and your partner did, you should let them go? If the partner knew from the start you didn't want DC then there is no responsibility on you to 'let him go'.
As I said, I used to always say exactly what you said about people not getting into or staying in incompatible relationship. If the sexless relationship is problematic then they tend to follow a pretty universal experience, and it doesn't sound like OPs relationship is like that at all.