Why did "thou" become obsolete?

According to the following source it was the use of you used first mainly as a sign of respect and then both as plural and singular form of address to replace thou gradually. (From Etymonline)

Thou:

• Superseded in Middle English by plural form you (from a different root), but retained in certain dialects (e.g. early Quakers). The plural at first was used in addressing superior individuals, later also (to err on the side of propriety) strangers, and ultimately all equals. By c.1450 the use of thou to address inferiors gave it a tinge of insult unless addressed by parents to children, or intimates to one another. Hence the verb meaning "to use 'thou' to a person" (mid-15c.). Avaunt, caitiff, dost thou thou me! I am come of good kin, I tell thee! ["Hickscorner," c.1530]

• A brief history of the second person pronoun in English can be found here.

You:

• Pronunciation of you and the nominative form ye gradually merged from 14c.; the distinction between them passed out of general usage by 1600. Widespread use of French in England after 12c. gave English you the same association as French vous, and it began to drive out singular nominative thou, originally as a sign of respect (similar to the "royal we") when addressing superiors, then equals and strangers, and ultimately (by c.1575) becoming the general form of address. Through 13c. English also retained a dual pronoun ink "you two; your two selves; each other.

It's generally not very polite in normal usage to refer to people with "you", to the extent that there's an obvious way not to do so. (Apparently not just a Japanese thing.)

• So if there are 100 synonyms for referring to you within normal usage, very quickly the head or tail (the most polite word or the rudest word) will lose usage to a specialized meaning. After that fossilization, you are left with 99 synonyms. (<meta>Or.. one is left with 99 synonyms.)

• So if there are 99 synonyms for referring to you within normal usage, very quickly the head or tail (the most polite word or the rudest word) will lose usage to a specialized meaning. After that fossilization, you are left with 98 synonyms.

"Why will 'thou' become obsolete?"

Over time, there will only be one word that can be used to refer to "you" in normal usage, if any.