Leaving aside ice&fire's distortion of reality...
There were accusations of clinic-type things ruining his kidneys. He denies this, as he would whether or not they were true.
He claims that after the 83 Giro (he was ill for the last few days) digging so deep to keep the maglia rosa over the last very hard mountain stages and final TT affected him negatively for 84 and 85. It does fit the timeline as he did absolutely nothing in the 2nd half of 83 either.
In 86 he changed coach (and doctor) and had a resurgence, but then disappeared off the map for good in 87
He claims that because he had won everything he cared about - MSR, Lombardia, Worlds, Giro - and the italian teams wouldn't allow him to focus on the Tour or Vuelta*, he simply didn't care much anymore and was just collecting salary so he reverted to being the overweight lazy kid he had been as a teenager.
I watched RAI's "Dedicato a..." episode on him specifically to hear what he'd say about it, as I'm similarly curious, but Bulbarelli didn't press him much on that point
The Giro did finally get harder from 86 onward, and that would've made it far harder for him to win it, but it definitely wasn't the reason for his massive decline in all types of races.
*An aside on italian cycling during this time.
There were a huge number of italian teams during that period, all with small squads and full focus on italian races. Several had exactly the required squad of 10 starters + 1 reserve necessary to ride the Giro.
Moser would publicly ask to ride the Tour every year and be denied every year. So Saronni was limited to starting one Vuelta as Giro prep, being allowed by the team 1 stage of his choosing to go for before retiring, murdering the field on that uphill finish, and then pulling out. Moser also did only 1 Vuelta, the year before that.
You'll notice that not a single italian was on the Tour GC top 10 during the entire 1980s. Few italian teams entered and the ones that did would either have foreign leaders or more commonly just use it as another race.