What impossible chain of events has lead to this?

Rating: 2/10 (19 December 2017)

Maybe a real movie critique should be harsher and expect more, but I gave The Force Awakens(from now on "TFA") 10/10 stars. I too saw that it was regurgitating A New Hope's plot and I did see that some of the humor was very J.J. Abrams (or even Temple of Doom'ish) and also that some of it was too fan-service'y, ...
but overall I thought it DID capture the spirit of Star Wars, had an amazing art direction, a brilliant and lovable cast of diverse main characters as well as a John Williams that finally created something very special again.
Sure, TFA's plot played it very safe and I have to assume that Disney hired Abrams to make one starter movie to set the right tone for all the movies to come, which he did brilliantly.
Now that I've talked too much about TFA, let me say that The Last Jedi is a gigantic failure and ignores the tone set by Abrams for the future of Star Wars under Disney.
What works:

The Acting: All the actors tried their best and gave good performances. Some even extraordinarily so like Mark Hamill and Adam Driver.
The Score: Williams did the main work for TFA already, but mixed it together very well again here to match the individual scenes and cuts.
The Effects: It's Industrial Light and Magic, of course they are great. It's Star Wars with lots of money behind it, so yes, there are lots of them.
The Art Direction: Not as strong as in TFA or Rogue One, some designs seem too aggressive like the new AT-ATs with their red eyes. They don't capture the Nazi design that the original saga was inspired by for the empire with white-black contrasts, concrete-colored greys and little but strong red. I also noticed a Rebel rifle prop that was clearly recognizable as a German G36 with just some added parts to make it look "space opera". The Art direction needs more time, more professional editing and less intervention from people who don't know what they're doing (like Last Jedi's director and screenwriter).

What doesn't work:

The Plot: The whole movie is based on one giant plot hole with many, MANY, M-A-N-Y smaller plot holes and misconceptions inside of it. Some of them related to physics, space, military tactics and military technology, which of course were always an afterthought in the Star Wars universe, but all the movies before managed a degree of coherence in this regard - like George Lucas made up a rule set of how things work and what degrees of liberty were allowed. The Last Jedi has ZERO understanding of coherence in sci-fi and fantasy.
The Tone: About 25% of the individual scenes hit the right tone. 50% were as off as Episode I and II and the last and sadly most impactful 25% were absolutely missing the mark, transforming the movie either into a comedy with jokes that didn't hit or a fantasy film with involuntary jokes that you *want* to laugh about at first but then can't, because you're that much in shock that this can happen in a usually test-screened and child-proofed to death, gigantic production effort like a modern Star Wars movie under Disney.
The Characters: Director and Screenwriter Rian Johnson doesn't understand who these characters are or how they would behave... no scratch that. Actually, maybe he DOES know, but chooses to write them like a self-indulged puppet master without coherent motivations and an inner self, giving up their individuality to support misguided theatrics and illogical interactions.
The Humor: Original Star Wars humor comes from the characters. Han Solo never fully takes a situation serious and this then leads to some funny scenes or character interactions. In the Last Jedi the humor is written like in a comedy. Characters and environmental actions don't result in humor naturally, they are artificially designed - against odds and coherent behaviors - to be funny, creating a great disconnect between the few scenes that do belong in a Star Wars movie in a way that suffocates any immersion in this world.

If you want a good comparison:
The Last Jedi is missing the mark for a coherent plot, tone and understanding of its characters as much as INDEPENDENCE DAY: RESURGENCE (though that one didn't even have any particularly great footsteps to follow). It's that bad!
Really, this movie is baffling and tragic to watch... and I, in all seriousness, asked myself if I was seeing the wrong cut of this movie, if there is some strongly edited cut (with the director and screenwriter suspended from duties) playing in another theater, which all the RottenTomatoes.com critics reviewed.
3 out of 10
(5 out of 10 if you don't care about Star Wars and just want to watch something trashy with lots of effects on the side, fully aware that it's modern Hollywood CGI junk food)