Of course you are right, the degradation of the universe started with Portable Ops. Avek is just a great art by Ash Wood, the coolest I know with Big Boss, and also suitable for forums - the game is tragic.
Well, in my opinion, MGS4 destroyed everything that was valuable in the series from a fictional perspective, answering every question asked by fans over the years with one word - nanomachines. The very running of the plot is the bottom line, I already preferred classic codec conversations than subsequent "lectures" that were given to us by key characters during the game. Ugh, come back; movie inserts. Ugh; slide shows. I will remain silent about Johnny and Meryl in the final act.
Let me put it this way. Over the years of dealing with the fandom, I have read many more satisfying theories and answers to the questions posed by the previous installments of the game, and I myself was the author of many. In my opinion, what Kojima did is a cheek to the fans because the level of plot we got compared to the previous parts and expectations is a mockery.
The problem is that in trying to end the story in a complementary way, the story has fallen into ridiculousness and shallowness. This is the problem that applies to all media productions that base their charm on mystery and riddles (LOST - Lost, is the best analogy), because when you start to answer them, there is nothing left but the viewer's profession. (By the way, I think LOST has chosen a better way, ending the story of the heroes, but leaving the island's secrets intact and to be figured out by any attentive viewer on his own.)
Okay, I guess that's a very subjective opinion after all. The objective thing is that even by Japanese standards and what the series has gotten us used to, part four is simply funny instead of serious by the scale of the absurd.
Now it's just the facts - gameplay, setting and performance.
As for the former, there is of course zero progress compared to the predecessors. Not making progress in this industry is going backwards.
Graphically, the game presented a good level at the time of its release, but we are only talking about graphics in the strict sense. However, in general, the eyes were struck by the disastrous animations straight from the early years of the previous generation (which in the three could still be forgiven) and the weak design and architecture of the environment. Why, it's a brick-and-mortar copy-and-paste game. Vampires with a Euclidean taint would not live long in the world of 90-degree angles, but Vampa probably saved nanomachines.
The technical execution is the bottom, the ubiquitous loading and these pathological installations revealed the absolute inability of the Kojima team to adapt to the new generation.
All in all, I am convinced that all positive opinions about this game are due only to Konami's efficient marketing, hype and fan sentiment, which overshadows the possibility of a reasonable assessment. So, ultimately, in the face of legitimate expectations, this game is a failure. If not for the three letters in the title, no one would even notice it.