But the chief reason of their failure--and this is, indeed, Browning's main point--is that each of them tried to do more than our limits on earth permit.
Paracelsus would have the whole sum of knowledge, Aprile nothing less than the whole of love, and, in this world.

It is impossible; yet, were it possible, could they have attained the sum of knowledge and of love on earth and been satisfied therewith, they would have shut out the infinite of knowledge and love beyond them in the divine land, and been, in their satisfaction, more hopelessly lost than they are in their present wretchedness.

Failure that leaves an unreached ideal before the soul is in reality a greater boon than success which thinks perfect satisfaction has been reached.

Their aim at perfection is right: what is wrong is their view that failure is ruin, and not a prophecy of a greater glory to come.

Could they have thought perfection were attained on earth--were they satisfied with anything this world can give, no longer stung with hunger for the infinite--all Paradise, with the illimitable glories, were closed to them! Few passages are more beautiful in English poetry than that in which Aprile narrates his youthful aspiration: how, loving all things infinitely, he wished to throw them into absolute beauty of form by means of all the arts, for the love of men, and receive from men love for having revealed beauty, and merge at last in God, the Eternal Love.
This was his huge aim, his full desire.
Few passages are more pathetic than that in which he tells his failure and its cause.