The altitude makes very damage. Col iseran is not the most difficult climb in the world (12 km at 7.5%), but look to the gaps that were made in the tour 2019, even with a weak field.I was two days ago on the bonette, it's a tough climb but no way comparable to granon or loze, besides the moutière paved section is quite horrible and I don't think they'll ever pave the remaining 2 Km as you're in the mercantour park, nature is protected..
The altitude of col de la bonette with his long distance, can make very damage.
Of course, i think granon is much better, but we can have more climbs with altitude in the tour.