Well I'll try again on Eternatus after doing a bit more calcing on it. In addition to the trouble w/ Calyrex-Ice that I tried to mention, it has a poor match-up vs. the other Great and Top tier legends, bar a locked Kyogre though they can Dynamax to be unlocked. It's also bad vs. Dialga and Ho-Oh.
Vs. Yveltal: Yveltal can 2hko w/ Dark Pulse and it's common item, LO. To avoid their faster possible 2hko it may be able to kill through use of 1 DP and then Sucker Punch.
Vs. Zacian-C: 2hkod, and Zacian outspeeds if Jolly. Adamant will be 2hkod but can Ohko after rocks some of the time.
Vs. Kyogre: Ice Beam is a 2hko. Eternatus resists the other 2 main attacks of Kyogre but it can Dynamax to get Max Hailstorm.
Vs. Necrozma DM: Flamethrower activates WP and then EQ Ohkos. Alternatively, a weak Dynamax Cannon(resisted) into Flamethrower is not a 2hko and Necrozma-DM can set up 2 DDs before OHKOing.
Vs. Calyrex-S: Psyshock 2hkos.
Vs. Calyrex-Ice: Glacial Lance ohkos, 75% chance after 1 Cosmic Power so the odd CP Eternatus can't set up very well vs. it.
Vs. Dialga: Most run Roar of Time for dynamax power I'm guessing. This ohkos w/ Modest just fine, and Dialga takes a neutral hit ok and more so if it's the 2nd most common, AV.
Vs. Ho-oh: Ho-oh is specially bulky and gets EQ. Slightly more than half of them run EQ,.
Other than that I can tell Xerneas sets up on Eternatus despite the poison weakness. It's a prime target for Ditto and sometimes ohkod by Landog(or always by Max Quake.) So I think Eternatus should be Good or Great rank not top tier.
There are a few misleading statements here about Eternatus.
First off, this post clearly is under the impression Eternatus is supposed to carry the defensive load on its own, and that games typically play out in a 1 v 1 legend scenario. This is far from the case, especially when considering Eternatus. Eternatus is designed to status stall or hit as hard as possible when it comes in, and when it has one of the bad matchups that you mention, it pivots out into one of the many stall teammates it has. Eternatus is fantastic because it doesn't need to hard commit with Dynamax to be successful, so switching to a teammate has far less consequences than other legends, bar Zacian, who can also do hit and run really well.
1. Yveltal goes even with Eternatus. There is a huge opportunity cost with dynamaxing Yveltal, because you really dont want to eat Cannon, and both are at risk of 2hkoing each other. Meteor beam at +1 ohkos Yveltal. Dark pulse into Sucker Punch still has play around the fact that most Eternatus run recover, meaning that Sucker punch is not a free click to get the 2hko with Yveltal.
2. Kyogre does not match up well with Eternatus. Kyogre being forced to lock into Ice beam is not good for it, and 100% hp Spout usually fails to 2hko even on the switch. If Kyogre dynamaxes it will be slower than Eternatus always, and eats the double damage Dynamax Cannon while STILL failing to OHKO all variants of Eternatus.
3. Zacian is obviously not a good matchup, which is why Eternatus often pairs with things like Zapdos and Ditto, which can heavily punish Zacian and win outright after copying or Dynamaxing. But Zacian is also probably the best legend in the game, so its no surprise that Eternatus struggles with it just like many others.
These are simply the most common legends, so I figured Id address that to begin with, to show your assessment of Eternatus even in the world of legend vs legend only is incorrect.
Now, lets get to the main benefit of Eternatus that you have not touched on: its extremely hard to switch into. Although Zacian can often switch in with impunity, Eternatus is a huge momentum boost whenever it has good matchups and forces opponents to switch. Between meteor beam, toxic predictions, or just raw DCannon, it is very hard to not lose momentum when facing it, and it slowly get to the point where Eternatus outright wins. Sure, it might "lose" to these legends you point out, but you don't address the huge benefit of Eternatus in that it is almost without a doubt the hardest of the restricted legends for non-legends to actually beat. Anytime you have a non-legend in, bar Tyranitar, you are gonna have a bad time against it. It absolutely demolishes them. Ferrothorn, Cinderace, Zapdos, Rillaboom, all the stall spam mons, very few of them actually want to go in against Eternatus. In a real game, and not theorymon calcs, Eternatus can switch into so many non-legends for free, fire off a hit, and if they try to switch in their legend, they often will get to a damage range where they no longer can rely on dynamax to survive. The fact that you are putting up calcs vs random legends like Dialga, and none against the non-legend titans, like Cinderace (whom it straight up beats), shows that you are neglecting a lot in your analysis of this Pokemon.
I can see arguments for the great tier, and depending on the metagame, it might belong there. Especially with Kyogre currently there, which might be an oversight by us atm (I think most of us would rather see Kyogre rise than Eternatus fall). But Eternatus is nowhere near the low level that any of the Pokemon are in Good tier, and suggesting it might be around that level is pretty ludicrous.
Shedinja
Yveltal: Top Tier -> Great
Eternatus: Great -> Top Tier
Kyogre: Great -> Top Tier
Calyrex Shadow Rider: Good -> Great
Zapdos: Top Tier to Zapdos Tier
in top tier, which has risen in popularity. What are your thoughts on this potential organization shift?
great -> good
good -> great