'Whosever': Why is this word going out of usage?

Whosever foot fits into the slipper, that woman will be my wife.

Though English is not my native language, due to years of reading, 'whosever' as a possessive came naturally to me in the above example.

Online search shows that the word is very rare and many dictionaries are getting rid of it (Oxford and Collins hasn't, yet). The common alternative given is 'whoever's'. But many users say that it's better to use 'whoever's' as a contraction of 'whoever is/has'.

Whoever's given that job, they better be good at it.

Is there an accepted usage of 'whoevers' (without the apostrophe)? Or, better yet, why not use 'whosever' more often in the right manner?

Solution 1:

Whosever:

• "Possessives. Whosever is the traditionally correct form, but is very much on the wane. Whoever's is now the preferred colloquial form. . . . Most strictly, whoever's is a contraction of whoever is (or, less commonly, whoever has)."

( Garner, Bryan A. Garner's Modern American Usage (3rd Ed.), p. 864. Oxford University Press, 2009.)

Whosever:

• In speech people sometimes try to treat the word “whoever” as two words when it’s used in the possessive form: “Whose-ever delicious plums those were in the refrigerator, I ate them.” Occasionally it’s even misspelled as “whoseever.” The standard form is “whoever’s,” as in “Whoever’s plums those were. . . .”

(Brians, Paul. Common Errors in English Usage. Washington State Unversity)