Why did the old pronouns and their respective endings vanish from daily usage?

Solution 1:

I'm sorry, but in questions of language, and especially of language change, "why" questions don't usually have answers, other than "because that's what happened".

The Early Modern English forms you give had already changed - in Old English, the pattern was very like modern German, but the distinct plural forms had mostly dropped out of use before Shakespeare's time (you still find ye goeth in the King James Bible).

The remaining distinctions are -est and -eth vs -s. The first of these disappeared when thou disappeared. The second was a dialect difference: IIRC -s was Northern, and -eth was Southern, though it may have been more complicated than that.

Edited to remove a wrong claim.

Solution 2:

In the European languages during the age of nobility, it was almost always a wise thing to flatter one's addressee with noble terminology, including subjunctives, plurals, third persons, abstractions like Reverence, and assorted short subjects.

Essentially, the idea went, nobles are more ethereal, more distant, more sensitive, more, ... um ... noble than you were, and therefore if you were singular, they were plural; if you were second person, they were third; if you were indicative, they were subjunctive; and so on. This is normal behavior.

Anyway, in Europe at the dawn of Modern European languages, nobility were important. Just as there were laws regulating who could wear the best clothes and styles, there were laws regulating how polite one had to be to which kind of nobleman, in deed and words. Especially words.

The results included

• Effective loss of 2sg pronouns and verb inflections in English -- replaced by the plural, erasing the number distinction -- as they were deemed unsuitable for polite company, and their use in public was taken as a mark of hidden Puritan sympathies. Or worse. This "plain speaking" got the Quakers banished to Pennsylvania, in part.

• Remnants of third person formality in phrases like ma dame/mon sieur, if Your/Her Ladyship would permit, Vuestra Merced etc.

• Gradual movement in High German from

  • Du bist [sg] - Ihr seid [pl] to
  • Du/Ihr [Informal sg/pl] and Er/Sie [Formal sg/pl] (Formal pronouns always capitalized) by 1800; (Tieck's plays are full of 2FormSg Er ist and Sie ist) to
  • Du/Ihr [Informal sg/pl] and Sie [Formal sg/pl] (Only Sie sind, always capitalized.)
    the modern norm, by 1850 or so.

• Use of third person Usted/Ustedes in Spanishes for 2Form sg/pl, with later local variants on vos(otros), and local extensions of Ustedes for all pl 2nd person, Formal and Informal.

It's a normal enough pattern, as I mentioned. This kind of stuff happens all the time, in all kinds of languages. Politeness turns out to be complicated.