Triploid apples don’t have trouble getting pollinated by diploids. The problem is that the Triploids cannot return the favor.
Imagine a scenario with two apple trees on a deserted island: one Gravenstein (triploid) and one Yellow Transparent (diploid).
Both apples are early-blossoming. Bugs on the island visit both trees’ flowers.
Yellow Transparent’s male pollen would fertilze Gravenstein’s flowers. But the pollen-sterile Gravenstein would not be able to fertilize Yellow Translarent’s flowers.
Therefore, every year, Gravenstein would bear apples, while Yellow Trans would not (assuming the insects do their thing). Yellow Trans would not bear apples because Gravenstein failed to fertilize it.
If a third tree were planted in proximity that happened to be another early-blossoming diploid (such as Black Oxford, for example), then ALL THREE trees would get pollinated and bear fruit. Black Oxford pollen would fertilize Yellow Trans flowers. And Yellow Trans pollen would fertilze Black Oxford flowers. Pollen from both Yellow Trans and Black Oxford would fertilize Gravenstein flowers.
Then… if the Yellow Trans tree died… the Black Oxford would discontinue producing apples in subsequent years because the sterile Gravenstein would continue to fail to pollinate anything.