The ten-based numerals пятьдеся́т, шестьдеся́т, се́мьдесят and во́семьдесят all ditch the final soft sign of де́сять and what decline as if they still retained it. If the declension remains unaffected, what purpose would it be to not have it at all?
The word десять historically was a noun (as were all numerals starting with five), but unlike пять, шесть etc. it had a consonant stem.
Words with a consonant stem had the ending -ъ in plural genitive in Old Russian.
Other words like this are:
- Those in -мя: имя / имён, знамя / знамён etc.
- корень / кореньев (< коренъ)
- (dialectal) день / дён (< дьнъ)
- мать / матерей, дочь / дочерей (< матеръ, дъчеръ)
and several others.
So десят (and сот) used to be the plural genitive forms of corresponding nouns and had been fossilized in the modern numerals.
Два, три, четыре were definitions and had to agree with the nouns they defined, that's why we have the двести < дъвѣ сътѣ (dual nom.) and триста < три съта (pl. nom.).
However, пять and above were nouns on their own, and their modifiers had to agree with them, not the other way around. This is similar to how English numerals like "couple" or "gross" work: you say "two eggs" but "a couple of eggs" or "a gross of eggs".
Redundant in light of Quassnoi's answer
I'd speculate that the component
десят in the numerals of tens must be not
десять bar the soft sign, but an inflected plural form of
десяток (or its cognate where the soft sign doesn't obtain), because grammatically compounds with numerals
пять - девять would require (at least by modern grammar) the 2nd word to inflect in plural Genitive. As one would say e.g.
пять раз where
раз is Genitive plural form of singular Nominative
The form десЯт with the stress on the last syllable is reminiscent of plural Genitive case inflection of such words as ребят, поросят, опят, чертят etc. which like
десят-ок in singular also in part have
реб-ён-ок, порос-ён-ок, оп-ён-ок, черт-ён-ок.
That however doesn't explain the soft sign in
тридцать but they also differ from the rest in both the reduced form of
дцать and the stress placement.