Riders struggle with 4 consecutive Gts. You are effectively completing 12 weeks of GC action in 52 weeks.
55 weeks for 'Tour-Vuelta-Giro-Tour';
62 weeks for 'Tour-Vuelta-Tour-Vuelta'
33-34 weeks break between Vuelta and Giro
Again, i'm not so convinced it's such a big factor:
If anything, I'd say the two GTs per year, in two consecutive years, aspect, is bigger.
But even more so, what matters is how tired you were at the end of the first GT of the year.
Those 8 months between Vuelta and Giro are a long break.
If you are feeling tired you might skip a ' Volta ao Algarve' or whatnot.
They don't need to race a lot to prepare, nowadays, anyway.