Ultimately, I guess that some series, by virtue of not 'progressing', become perpetual motion machines.
I agree with your whole long post, but this raises the question of (although I know quite well what the strict-constructionist purists are looking for) what exactly is “progress” in a series where the protagonists are together for life basically from the first chapter and who form the model yuri couple for all the budding lesbians around them.
I mean, your standard 'progresion purist' probably has a mindset that goes like this:
Chapter 1: If they don't confess, the manga's a mess.
Chapter 2: If they don't kiss, the manga's a miss.
Chapter 3: If they don't French, my fists will clench.
Chapter 4: If they don't get to second base, they're not worthy of my grace.
Chapter 5: If there's no cunnilingus, the author's just conning us.
Chapter 6: If they don't trib, I shall return to the comments and crib.
Chapter 7: If they don't cohabitate, I shall spew forth torrents of hate.
Chapter 8: If I don't see a wedding, there shall be a beheading.
On a more serious note, I'd say the desire for marked, clear 'progress' comes from a tendency to view relationships like videogames or TV shows, where there's always something better or new waiting as you get further in. The entertainment industry is tasked with fetishing and glorifying romance for vast numbers of consumers, the majority of which have not entered into romantic relationships by themselves. Portraying romance in an understated, pragmatic, subtle way wouldn't lend itself well to a seasonal entertainment system or a three-act story structure, even if real-world romance is rarely so linear.
Audiences want to see melodrama, overflowing emotions, dramatic confessions at airports right before planes take off, wonderful sex between beautiful people, and the longer they're invested in a relationship, the more dividends they expect to gain from it. This, I'd argue, is primarily a Western phenomenon, since the SOL genre does not exist in Western television, or is at least not nearly as popular as it is in Japan. Most of the readers on this English forum are presumably Western, and have thus grown up consuming stuff churned out by Hollywood.
These ideas, which initially began as entertainment, start to be taken as common knowledge and become actively aspirational, so if you're not rolling in romantic partners and having orgies by the time you're twenty-five, you've basically failed in life by the standards of popular entertainment. The rise of incels in America, for instance, could be directly linked to the increasing sexualisation of American culture and media, which idealizes sex as an activity between young and beautiful people, leading those with inferior self-images to construct a warped world-view. However, if all it took to have the ideal relationship was to be conventionally attractive, then the love lives of celebrities wouldn't be anywhere near as messy as they are.
Coming back to this manga, I'd say that these progression-purisits are people who fundamentally seem to engage with fictional romance as an investment-reward game, and want to receive repeated 'signs' that the couple's life is better than it used to be in order to justify consuming more of the manga. They think that there must always be something more, something further up the ladder, like those weird rituals about wedding anniversaries that ask you to buy gifts of silver and gold and platinum to mark the progress of matrimony.
But this mindset fails to take into account the joys of stability, of domesticity, and of simply coexisting. Life isn't a rat race to the next level of romance or sex, it's a swirling, insane whirlwind of unpredictable bullshit that's likely to either crush you with monotony or tear you out of your comfort zone on a dime. In such a world, there's an incredible, heartwarming appeal to stories about stability and peace, about subtle affection that doesn't need to be over-the-top or escalating to justify it's existence. It's about people who don't need to explicitly say that they love each other, just as normal people might not thank their sidewalks for not collapsing into a yawning abyss, because that's to be expected. That's the golden, glittering status quo.
I'm not saying that such stories are realistic- having your first love transform into a lifelong romance is every bit as farfetched as cultivating a harem at the age of sixteen. But to audiences, particularly in Japan, that got shunted out of soul-crushingly competitive school lives and into a mind-numbingly oppressive job environment, these series provide the sustenance of a rose-tinted adolesence that they never got to enjoy, an adolesence where they never had to prove anything, but could just be themselves, sequestered from spacetime in a cozy corner of the world.
There's no need for filling out a mandated sex quota to keep the spark going, no need to do something unpredictable or new, no need to introduce scandalous and sensational twists, because for the target demographic of such stories, tomorrow being the exact same as yesterday isn't a tragedy- it's the most wonderful promise you could ever make. And unless people take that into account, they probably won't be able to enjoy A Room for Two.