At this point, even if a TPM can recreate much of Pluton's functionality, I still believe some fear regarding Pluton is still necessary and healthy, although I do not dispute that for some uses it may be useful - after all, why was my fear mongering section explicitly labeled "Fearmongering and Doomsday speculations"? Microsoft can still screw people over, but Pluton is different from a TPM and should still be (generally) regarded with caution where possible, and more caution than a standard TPM.
This is mainly because, at this point,
A. A TPM's level of access and capabilities to a system is well-known at this point. Pluton, we do not know with certainty what all of its capabilities are.
B. Microsoft has explicitly stated Pluton will have functionality added to it in the future though software updates, most likely that cannot be downgraded, that are not present yet. It's not that Pluton might have stuff added later - Microsoft has said stuff will be added later. What these upgrades entail or are capable of is also unknown.
C. Because of the above, Pluton requires a previously-unknown level of trust for Microsoft, because Pluton almost certainly has anti-downgrade procedures. Microsoft could, potentially, send out an update just blocking Linux and if Pluton received the update, it would be irreversible. Maybe this isn't within Pluton's abilities, but we just don't know. Just that Microsoft (or a hacker of Microsoft - I'm more concerned about a rogue employee than Microsoft at the moment) could have permanent effects on the security of a system is worth paying attention over.
D. Because of the reasons above, Pluton should be regarded with extra skepticism as it is a magical black box, with unknown capabilities, that it is not clear whether it can actually be disabled. (Already on my blog, there's a user talking about how Pluton briefly boots and then disables itself if the UEFI says that it should be disabled, not that it never starts, so theoretically a Pluton update could ignore its own disable switch.) I don't have verification of that, but until we know more... TPM is known, TPM can screw people, Pluton has the potential to extremely screw people over, and while many of my doomsday speculations can actually be recreated with just a TPM if TPMs are widely adopted, perhaps it could be enhanced with more Pluton-specific ones. Perhaps my doomsday predictions actually weren't far enough.
Thus, your point that Pluton doesn't add too much might be completely valid right now. That doesn't mean Pluton isn't also a potential Trojan horse that Microsoft updates as they please with new things that we didn't expect or ask for with no ability to undo them.
Edit: Removed a previous edit, and adding that, to complement the above notes, it does not help instill confidence that Microsoft isn't telling what Pluton can and cannot do at a hardware level. They've said a few things it can do right now, and just said more stuff will be coming in the future, but they won't talk about where its limits are. So... trust the black box without questions please. To be fair, this isn't the first time (Intel ME, AMD PSP?), but it is unsettling to have another one.