They did not intend to conquer England in the first place. They wanted to depose the (heretic) bastard Queen, restoring the One True Faith (tm.), and end English support to the Netherland rebels.
In ther end, even had they won, when they left they probably would have only managed the first part (with the certainnty of one or two Elizabethan pretender rebellions down the line for whoeeve the new king was...), and restore catholicism for one generation at most. The netherlands would have been at most three or five years of respite.