I think you're seriously underestimating the effects of Roman meddling in Seleucid affairs and the damage it done. Here's my posts from another forum regarding that subject:
"Interesting Stark, but didn't they lose Babylon and Mesopotamia to the Parthians pretty soon after that?
So do you think that the real reason that the Seleucids lost Iran and Mesopotamia to the Parthians was because they had already been weakened by Rome's meddling?
Depends what's soon for you. Seleucid mint in Ekbatana was still functioning during rule of Demetrius I, and text from Susa were dated by Seleucid era until 142 BC. So it seems they held on to atleast western Iran by that time, some 40-50 years after Peace of Apameia.
About Roman meddling, I think if they remain completely out of the picture, yes, seleucids have a better chance than Parthians to remain an eastern rival to Rome.
Look at the situation before Peace of Apameia - Antiochus III humbled Parthians and Baktria during his eastern campaign (of course this was just a temporary state, dependant on future strenght of Seleucid state), neutered Ptolemaic kingdom at Panion (their phalanx corps was destroyed there and never rebuilt), after which he took control of entire Levantine coast, and eliminated most of their holdings in Asia Minor. Pretty good situation for Seleucids.
After that he gets crushed by Romans, losing all territory beyond Taurus, and has to pay large indemnities (collecting which gets him killed). So, without Romans we have stable (for now), rich and powerfull kingdom without any major rivals.
But, even if we remove Romans after they defeated Antiochus, it's still pretty helpfull. After death of son of Antiochus III, Seleucus IV, Romans back Antiochus IV (younger son of Antiochus III and their captive) as ruler instead of legal heir, Seleucus son Demetrius, who instead goes to Rome as hostage. Antiochus IV was, even after losses dealt by Romans, on the verge of conquering Egypt when he was threated by war by Roman emissary if he doesn't retreat, which he did. And it's important that this happened just after Battle of Pydna - if Romans we're still engaged in Macedonia they might not feel strong enough to threaten Antiochus, or he might ignore them. Egypt, insanely rich province, would do much to strengten hellenistic power in the east, but that was not to be.
After his death, Romans witheld their captive, 22 year old Demetrius, from attaining the throne, so a 9 year old son of Antiochus IV ascends to the throne. During his reign, Romans use his weak rule to impose demilitarising terms of Treaty of Apameia, sending consul Gnaeus Octavius to "burn the decked ships and hamstring the elephants". This was so unpopular to the public that consul was latter killed in Laodicea.
After that Demetrius finally escapes Roman captivity and takes the throne, but is later defeated by Roman backed pretender Alexander Balas, reportedly useless and decadent ruler dependent on Ptolemaic support.
Also, in "From Samarkand to Sardis", author Susan M. Sherwin claims Romans backed Maccabean revolt, though she gives not sources on this so I'm not sure if it's true.
My final opinion is, Romans inflicted enogh material losses and damaged dynastic stability and prestige to critically weaken Seleucid state in it's attempts to retain their eastern territories, a task hard enough even without outside interference."