Does the use of modern contraceptives such as pills and condoms, or the invention of a drug that prevents pregnancy one hundred percent, permit Zina (unlawful sexual intercourse), especially if society does not consider it a breach of chastity and dignity?

What you mentioned about the mixing of lineages and the proliferation of foundlings are among the wisdoms behind the prohibition of fornication, not the cause of the prohibition. The cause of the prohibition is that fornication is an abomination and an evil way, as stated in the Noble Quran: "And do not approach unlawful sexual intercourse. Indeed, it is ever an abomination and is evil as a way." The jurisprudential rule is that a ruling revolves around its cause in existence and non-existence, not around its wisdom. All divine laws and sound innate dispositions deem fornication as ugly and having evil consequences, and all divine laws have forbidden it. Even Arabs before Islam found it detestable. Societies whose innate dispositions have been inverted consider the ugly to be good and the good to be ugly, but this does not change Allah's law. Allah Almighty said: "And if the truth had followed their inclinations, the heavens and the earth and whoever is in them would have been corrupted. Rather, We have brought them their reminder, but they are turning away from their reminder."