Is modern Megaten in a "Zombie Simpsons" phase?
Yes, although I think the same of any franchise that has shambled on for over 35 years. Everything endures so long as to become the proverbial snake eating its own tail, the object inspired by itself, a procession of familiar symbols evoked over and over again and diminished with every rehearsal. Most series will reach the point where they can no longer tell you anything meaningful about the world because they can only speak of themselves. Should the notion of another Tokyo touched by apocalypse inspire you? How could it? The staid and aging userbase for nearly all legacy franchises points to a logical endpoint in the final senility of both IP and audience, but the road that leads there will be very long and very, very boring.
SMT is, like FF, in a more pliable position than most, since the self-contained nature of each game precludes the worst excesses of continuous timelines and 'lore'. But as of the last decade or so the series has largely ceased to absorb new and distinct influence from outside of itself, holding fast to the most suffocating elements (Tokyo as setting/axis mundi, LNC but especially LNC organized under the same Abrahamic schema) while regarding with disinterest its lifeblood (an earnest if unrigorous approach to myth that did not treat the subject as an obstacle to maneuver, as with the Nahobino concept and its subordination of mythical forms).
To its credit, SMT is backwards-looking in a very ordinary way. It avoids the total collapse into metanarrative evident in, say, FFVIIR, and neither has it begun to regurgitate itself in the manner of Resident Evil and its program of remakes. And nothing can match The Simpsons proper, where both the persistence of long-obsolete archetypes (Disco Stu, Comic Book Guy) and the tortured floating of the timeline have lent the thing a genuine, anthropological fascination.*
Following the videogame industry for over two decades has radicalized me against the mere concept of franchise. But if all franchises were to be abolished by holy writ, a host of Mighty No. 9 type simulacra would replace them on the following day. Our imagination is arrested with the inertia of capital, and nothing will be put to rest with dignity. No investor will say, well, this thematic space has been explored to satiety- time to move on! And we've learned to speak their language. Nothing is more damning than the ease with which we use the term 'IP', as I did earlier in this post.
The ideal outcome for any senile franchise is to explode after a fantastic, rupturing experiment, like Breath of Fire: Dragon Quarter. The least desireable outcome is to endure as a homunculoid GaaS project, like Breath of Fire VI. If SMTVI emulates the former and assumes a shape so jarring to its pedigree that it kills the franchise, then I'll take that. If it turns out to be Nocturne but with boobies then I'll take that too. Presumably it'll be some other, third thing, but I'll take any garbage they're willing to throw at me. I probably won't be paying for it, though.
*The one point that I'll admit in favor of franchise is that it acts as a neat conduit for this phenomena. Everything gains interest with age. Any appropriately aged art or media, in addition to whatever distinct artistic merit it might possess, is shot through with the historical conditions that produced it. And being produced continuously, a sufficiently long franchise can channel the alterations of time insofar as different iterations distort and color the same basic container. Think of how Star Trek models the same premise across its various decades of existence.
Of course, even Ant-Man and the Wasp will be an object of rich anthropological interest one hundred years hence, but I'm not sure if it will ever be a vital piece of art. So too with the lowly franchise.