That's under the MASSIVE assumption that DL is a manga that has such a singular concept as OPM. The reason that anybody would be turned off by the shift in focus away from Saitama and onto other characters is because Saitama is the singularity that the series hinges on; he is THE One Punch Man. Saitama is what makes the concept that OPM is built on function at all. He is so simple and so simply strong that he can carry a series just like that and fulfill the fantasy of strength in the readers. However, conceptually, Saitama only works if every other creature is treated as another jobber that serves as a measuring stick of Saitama's strength. In this case, you are right. In ONE's decision to give a spotlight to the rest of the case and make them real characters with their own strength outside of Saitama, the series stopped being about its singular concept of an ordinary man who yearned for incredible strength, only to achieve and grow bored. It, as a story, fundamentally changed from the point ONE shifted the focus to more heroes. In this way, people who chose to read the series to fulfill a fantasy of Saitama's strength may find themselves alienated as they lose the place to project themselves onto that vessel of strength.
However, the change in the fictional manga of DL was not a fundamental shift like that, but the fleshing out of a side character previously of little importance (but with some amount of a fanbase). It is not in the same vein as OPM expanding its scope but in the same vein as something like the Kuma flashback in One Piece. One Piece has always been a story about dreams and freedom and even rebellion to a certain degree, the fantasy being sold through Luffy's pursuit of an impossible dream, but Kuma in many ways is an antithesis to this who does not dream of greatness, decided to submit to authority, and lets himself be bound by others. However, there isn't a dissonance between the feel of Kuma and the feel of the main plot with Luffy (at least the serious moments), because One Piece has always been a series that valued the connections between others greatly, even to a greater degree than freedom or dreams at times. The central draw of One Piece, the fantasy that is being fulfilled, is almost certainly the romance of adventure and freedom and the pursuit of dreams. But time and time again there are character's bound to others or to duty and who will give up parts of their own joy and wellbeing for the things they are bound to (including and especially Luffy). Under an understanding of the central draw/fantasy to be fulfilled being tantamount to its merit as a work and main idea, characters who will choose to be less than free for the sake of something else being so idealized would be a mistake on Oda's part. A mistake he has repeated over and over and over again, ever since Chapter 1. For this reason, I view this fulfillment view of storytelling as very shallow. One Piece has always been a story about someone who lives freely and someone who lives true to his dreams, and it has also always been a story about kindness and bonds. The fantasy is freedom, but the central idea that underscores most of the positive moments is kindness and love (and what underscores negative moments is contrarily cruelty and hatred).
Manga is a special medium in that it is both entertainment and art. There is absolutely nothing wrong with reading manga to fulfill a yearning missing from your life or reality itself. However, the fulfillment of a fantasy is nothing but a singular independent but interconnected aspect of a work. The core fantasy and the core ideas presented in a manga are inherently separate and capable of being changed independent of each other (although most works will likely align the two very closely). One Punch Man has always been a manga about an overly powerful but otherwise ordinary man destroying everything in his path, that was the hook, especially as a gag manga, but it has also always been about putting in the effort to achieve your dreams, the desire to challenge yourself, and the pain of stagnation or supposed stagnation. You can call the shift in OPM as a betrayal of the readers who were fulfilling their fantasy through a gag manga, but only because that fantasy being sold was in service to being a gag manga (and made an exploration of these ideas much harder). It is only in those cases where an author decides to fundamentally shift the tone of a story out of their desire to make their work about the ideas they put into it and not the fantasy they were selling that you could ever reasonably call it a "betrayal." And even then only when that core fantasy and the core ideas were at odds in some way would anyone decide to do so.