"The cost of not running Pdon"
Let's see... Grass weakness hmmmm that means... oh wait, only threat is Grasseus... who uses that when serious?
Water weakness... Kyogre and Porge and Waterceus? Pdon can lose to those anyways, also Groudon running Twave removes them from the competition if played correctly, also Since when does Pdon stay in for those?
Desolate Land + Harsh Sun removing water weakness... Those are the only pokemon that run water moves...
One less pokemon you have to choose an item for... I guess that's helpful if you are only playing for fun or on time limit.
+20 defense to an already high defense, which is higher than pretty much every respectable tank/stall mon with the exception of Megabro
+30 Attack.. it's base atk is only 10 lower than mega blaziken, the same as Zekrom, same as Rayquaza which already does a heavy chunk of damage. It also gets swords dance... Just because it's more powerful at a glance doesn't mean it's necessarily better.
90 base speed and rock polish so you can sweep. Groudon has that too...
A Ground weakness so you can get OHKO-ed by a +2 Ultra Necrozma or every Primal Groudon other than mixed one shots you so you better hope you are faster or mixed. Or you could just be mono ground type and survive a +2 Physical Unecro and either Paralyze it or deal massive damage to it to revenge kill and you can just survive Primal Groudon to one shot it.
A reliability on fire type moves because it's boosted... Although this one is pretty good to have, games shouldn't be about sheer force or OHKOing to win, but instead should have strategy and be fun for both players. Not saying this is busted, but it makes a norm even though last I checked, a fire type 2.25x damage bonus doesn't do anything but help against flying types and scares Ferrothorn. But wait, didn't people used to use Stone Edge back in the old days?
A switch-in to Xerneas. This is actually a benefit, but from what I have seen, just because something checks a mon doesn't make it better or more viable than another. Also Necrozma Dusk Mane exists.
There is more to cover but.. I think you got the gist.
Explain to me why is the opportunity too great that it makes Groudon unviable because it doesn't compare to another pokemon. If Pdon didn't exist, it would be considered viable but not as much as it would be now.
See, the problem with Groudon isn't that you're running Groudon. The problem with Groudon is that you
aren't running Primal Groudon, who leverages an absurdly good typing with an absurdly good ability and its absurdly good offensive stats and its absurdly good physical bulk and absurdly good movepool.
If Primals didn't exist Groudon would most likely still be a high or top-tier threat and Kyogre would most likely be one of the kings of Ubers again, but that Ground/Fire typing paired with its 180/150 offenses, its Thunder Wave and Will-o-Wisp immunities, and its Water pseudo-immunity just allow it to double-down on almost everything Groudon is good barring vanilla Groudon's Dragonium set (which I'll admit is actually pretty good and should push it to C- or C at the very least, alongside vanilla Kyogre) while giving it a laundry list of new options that vanilla Groudon couldn't even
dream of:
Want to blow your opponents back with a STAB, sun-boosted Fire Blast, Overheat, or even Eruption coming off of a nutty base 150 SpA stat? You can do that.
Want to clap Mega Salamence and a weakened Zygarde with HP Ice without having to blow a Z-move? You can do that now, and you don't give a shit about Intimidate from pre-Mega Salamence and you hit both on their weaker defensive stat to boot.
Want to use that superior typing and bulk to spread status or increase your setup opportunities tenfold? You can do that.
Want to hit harder than a standard vanilla Groudon with your physical sets, scoring several crucial OHKOs and 2HKOs? You can do that.
Want to run a double-dance set with your amazing dual STABs? You can do that since Ground/Fire is only resisted by a handful of notable Ubers.
Want to run a mixed set, which vanilla Groudon couldn't even
hope to do? You can do that.
Want to actually check Xerneas instead of getting blown up by it? You can do that, and you can do that even better when you pair it with Dusk Mane.
Granted, you do get OHKOed and 2HKOed by a bunch of Earthquakes flying around in this tier. But why do you you think everything and its mother is packing a Ground type move in the first place? It's not like the mon with 80% usage and one of its best partners seen on almost all team archetypes at over 50% usage are both weak to Ground or anything, right?
You're
seriously undervaluing Primal Groudon's effectiveness in this meta. There's a reason why Kyogre went from top 5 in usage to fringe viability the instant OR/AS was released. There's a reason why it has 80% usage. There's a reason why it has a set for literally every occasion and is held back only by what the rest of the team cannot cover for it. There's a reason why it has its own subrank dedicated solely to it. And there's a reason why that'll probably never change for at
least the next two generations. Having something with that much sheer power, defensive utility, and unparalleled versatility up its sleeve on its own is fucking incredible, and the icing on the cake and why it's just so much better than regular Groudon is because it neither takes up a Mega slot or a Z-move slot to do so. You can still have your Ultra Necrozma or Z-Move Dusk Mane, Mega Diancie/Mega Gengar/Mega Mence/Mega Lucario,
and Primal Groudon on a single team, and there is virtually nothing more terrifying than an offensive backbone of that caliber.
