the main character was time.
the time spent dreaming, cooking, waiting, worrying, playing, competing, eating, folding paper cranes, writing letters, keeping diaries, studying, being together, being apart, hesitating, acting on your instinct, running against, giving time, winning time, losing time, counting the times.

It's really not your fail at all, but as you did notice, kdramas tend to be slaves to certain storylines/story patterns for their own sake, and one of those is the rule that the guy who has a lot of screen time at the start, is probably the lead (this is also true of shoujo manga btw). I'm guilty of falling for that one myself, for that matter.

You're right that the boys had mostly equal screen time (barring Dong-ryung, whose moments in focus were sparser) but if you notice, the first few episodes mainly focused on Jung-hwan and Sun-woo, out of the boys - Taek was much less of a presence. And after Sun-woo's crush on Bora was revealed, people assumed that the other boy with the most screen time and the apparent crush on Deok-sun, MUST be the husband, never mind what happened later in the show.

But it's not just slavish trope-following that was to blame for this one - a lot of people, including me btw, took the 2015 husband's manner of speech, body language and mannerisms to be very similar to Jung-hwan's. That wasn't unreasonable either, since 2015 Husband seemed most like he could have been either Jung-hwan or Sun-woo, and after Sun-woo was ruled out, that left just Jung-hwan. There was no trace of Taek's personality or mannerisms in those early episodes, which really did confuse people since Taek's personality is such a singular one. That was a PD/writer/actor fail, and is almost as much to blame for viewer dissatisfaction, as the trope expectations.

I do agree that a lot of kdramas with the 'this guy gets the girl, just because!' mentality are a pain to watch, because more often than not the pairing isn't a result of organic storytelling or character development, it is, as you said, a 'just because'. Because so-and-so actor or actress has been hired to play 'the lead' and one of the hallmarks of a lead is, you must get the guy/girl otherwise it's an insult to their star status. Or because it's expected that a drama must have romance somewhere. It's rather rare for dramas to ditch that mentality.

@A separate blue
If Reply88 was my first kdrama I watched, I would totally feel confused or put R88 as my 1st favorite, since I'm always melted by a slice-of-life drama with good/realistic characters. But, these are my reasons why I love this show, and feel okay with the ending.

I didn't read Reply88 recaps or people's comments/discussions about it (like I always do with previous dramas), because I really wanted to enjoy all things I could get from the show "alone". I didn't expect anything from the love story. I told myself that I would try to see a bigger picture from anything, so I could be more thankful than be angry(?) when things didn't go as I'd been expecting.

When I accidentally read the spoiler of the endgame, I was upset, not because of the spoiler, but it's because I couldn't be surprised anymore like I used to. lol. (never cared about that husband hunting anyway). I love all the characters there. and, when I watched the finale, I was really sad over the fact that I can't see them anymore on my screen. I was really attached to them as a whole Reply 88 family.

This is not my first kdrama with beautiful characters, so it isn't my most favorite (Reply 1997 is the 1st on my list) ...but like you said, R88 taught us so many things.
I get why people focused more on Jungpal's closure (He won everyone's heart, including mine), but It's just sad that Reply88 is considered as a terrible show because of the last two episodes (or other reasons which will take many pages to write them down).
Well, I agree, it's screwed (I do think that there are many elements unsatisfying there), but Reply88 has been giving heartwarming reminders for me to be more kind to people, especially to my own family, since we don't know when we'll leave this world.

Probably, because I expected nothing but I got more(?). Or maybe I simply like it despite the flaws the drama has, so I get a very nice memory with Reply '88 :)