One of the things Hughes says (possibly in his foreword to his heavily expurgated The Journals of Sylvia Plath in 1982) which struck me as both terribly naive and, looking back from 2025, deeply old-fashioned, is something like 'I would hope that each one of of us owns the facts of his own life'.
Alas for him, when those facts are closely related to the facts of someone else's life, and that person is both celebrated and beloved, and dead by their own hand, AND a brilliant, copious and self-mythologising writer in her own right, he doesn't 'own' them in the slightest.
I think his destruction of Plath's last journal was an attempt to wrest back control of some, at least, of those facts by permanently denying access to them, but it's hardly coincidental that much of the destroyed journal dealt with his total inability to keep it in his trousers. (And this is an established 'fact', not just the speculation of a betrayed wife -- he wasn't even with Assia Wevill the night Plath died, he was shagging a Faber secretary called Susan Alliston. AW had to 'share' him with two other women. He was sleeping with someone else a few days after he married his second wife Carol Orchard. He told one of these women that he felt it was suffocating to be dependent on only one woman.)
There's a conflict of interest, ultimately, between the husband/soon-to-be-ex-husband (angry, grieving, conflicted, guilty, dealing with two tiny children) and the literary executor, in charge of preserving and promoting Plath's legacy, publishing work left unpublished at her death, ordering poems in Ariel, preserving diaries, drafts etc. (Note that while he badmouthed literary scholars and biographers all his life as 'vampires', he preserved all of his own diaries and letters and drafts, selling many to Emory University whose Coca-Cola endowment meant they had huge amounts of cash, hence TH made a lot of money,. His estate sold more material to the BL for half a million £. He definitely wanted them to be read).
The whole Plath/Hughes legacy story comes down in part to a tangled story of literary executorships entangled with emotions. His widow Carol refused to let his biographer Jonathan Bate quote from his work after initially supporting the biography, presumably because she didn't like what he was saying.
There's also the considerable baleful influence of Olwyn Hughes, Ted's sister, who was Plath's de facto literary executor, very protective of her brother, and who'd disliked Plath intensely on the comparatively few times they'd spent time together, had a huge fight on their last meeting in 1960, and who came to live with him to help bring up the children after Plath's suicide. I warmly recommend Janet Malcolm's book The Silent Woman, which is about different people's memories of/mythologies of Plath, especially Anne Stevenson's Plath biography Bitter Fame, written with Olwyn looking over her shoulder and continually interfering. Olwyn thought Plath was an awful, rude American princess with a 'difficult personality', no manners, and sick in the head, who killed herself out of spite and she was vitriolic to anyone who didn't share that view. According to Hughes' biographer Jonathan Bate, she softened her position after her nephew, Plath and Hughes's son Nicholas died by suicide in 2009. Then she started to believe depression was real.
As regarding both Plath and Hughes' stature as poets, I think it feels like a difficult comparison when he had such a long career and she died so young. I think her best work, though there's comparatively little of it, stands up to anything he wrote. I think Hughes was at times brilliant, but also enormously uneven, as you would expect from someone who wrote so much, and there are real blips (like Shakespeare and the Goddess of Complete Being, and some of his royal ass-kissing, to mention only two). It's hard to know what she would have been writing by the time of life he was writing Birthday Letters. I think TH was wrong to think her fiction was inferior. I think she would have written more novels, and they may have been extraordinary.