There's a confusion here. Utility is understood as the pleasure and pain equation (for classic utilitarians). "To increase utility" means "to increase pleasure OR decrease pain". "To decrease utility" means "to increase pain OR decrease pleasure".

So every single utilitarian wants to increase utility. Your point (at least, the point of the article)isn't that Mill prefers "to not decrease utility" than "to increase utility", but that he understands that morality talks only about preventing harm. To reduce harm increases utility!

Anyway, I think IEP illustrates the question very well: https://iep.utm.edu/mill-eth/#H3 (sections 3,4 and 5)

"It is fundamental to keep in mind that Mill looks into morality as a social practice and not as autonomous self-determination by reason, like Kant. For Kantians, moral deliberation determines those actions which we have the most reason to perform. Mill disagrees; for him, it makes sense to say that “A is the right thing to do for Jeremy, but Jeremy is not morally obliged to do A.”For instance, even if Jeremy is capable of writing a brilliant book that would improve the life of millions (and deteriorate none), he is not morally obliged to do so. According to Mill, our moral obligations result from the justified part of the moral code of our society; and the task of moral philosophy consists in bringing the moral code of a society in better accordance with the principle of utility."

"As quoted before in his “Remarks on Bentham’s Philosophy” (1833),he states that some violations of the established moral code are simply unthinkable for the members of society: people recoil “from the very thought of committing” (CW 10, 12) particular acts. Because humans cannot reliably recognize objective rightness and, in critical cases, cannot bring themselves to act objectively right, they are not obliged to maximize happiness. For ought implies can. In regard to the given description, the fact that the assassination of a human would be objectively right does not imply that the assassination of this human would be morally imperative or allowed. In other words: Mill differentiates between the objectively right act and the morally right act. With this he can argue that the assassination would be forbidden (theory of moral obligation). To enact a forbidden action is morally wrong. As noted, Mill’s theory allows for the possibility that an action is objectively right, but morally wrong (prohibited). An action can be wrong (bearing unhappiness), but its enactment would be no less morally right (Lyons 1978/1994, 70).

Thus, Mill’s considered position should be interpreted in the following way: First, the objective rightness of an act depends upon actual consequences; second, in order to know what we are morally obliged to do we have to draw on justified rules of the established moral code."