I actually agree with many of your criticisms of the movies and Jesse's character. With the posts in here about the movies it feels a bit like I still can't be bothered with them and I still adore the movies anyway, and I have been trying to think of reasons I can verbalize more. Something like...
Before Sunrise: I am not sure it comes down to more than me loving the idea of people having these brief, powerful encounters. I thought it was refreshing for the genre to just have a movie about a connection like that, and also leaving us with an ambiguous ending. Our human interactions are way more like these, but movies in this genre are populated with the impulse to make everything being happily ever after. On repeat viewings it's my least favorite, but I still love many of the scenes in the movie. I was happy not knowing and having that discussion with others who saw it, and had no desire to see a sequel. Well, right up to the point where it was announced that there was a sequel.
Before Sunset: My favorite of the 3, and a favorite all-time movie. I don't love the flashbacks at the start, but once they interact, I am 10000% on board. What I think elevates this one even more is the decisions to let them write a lot of their dialogue - it seemed to really be a 3 person passion project now. Also, the ticking clock of needing to catch a flight and watching the encounter in real time is dynamite. What I also love is that it does become a bit of "careful what you wish for" for both the audience and characters. There are other people involved, and their decisions have even more ramifications now. Of course my heart wants something when the Celine's last line is delivered, but on repeat viewings there is even more weight there now.
Before Midnight: ...and I think those consequences I spoke of bubble up in this movie. Not as hopeful, not as romantic, but I still appreciate where it went. It goes to realistic places based on the choices they have made at the end of Sunset. I agree with Eephus above and did struggle with the ending on a rewatch. In my world and experience, what Celine says to Jesse would have much different effect on me, and it does seem a bit odd that his goofy pickup lines would save the day. I also could have done without the dinner party.
Still, as a whole I think they work incredibly well together, and I have always appreciated the love and thought these 3 put into the characters and these movies. I'm not sure there needs to be any more, but I would still look forward to seeing what they came up with. My understanding is they toyed with the idea around the same 9 year mark, but simply couldn't come up with anything they thought was good enough. Again, I appreciate that and the fact they didn't just plop one out just because.