Oh really? On the day that English was born, English grammar was all intact and just needed more words to grow. I did not know that.
The grammar of any language is all intact at any one time, but not static. All living languages undergo change all the time. English hasn't been getting any better, just different. The change has no destination; it just changes.
English is one of the most loan-word heavy languages on earth, but nearly all our prepositions, pronouns, and articles - the most-used words we have - were already present in English when English was Anglo-Saxon. That's not true for verbs and in particular nouns; they're what English borrows. You can take a some foreign nouns and verbs, arrange them with english prepositions and verb tense markers and all of a sudden you're speaking good English: [The maitre'd segued into a bravura recitative concerning the table d'hôte menu
or The loco hombre's pinto cantered into the arroyo.
]. The nouns, verbs and adjectives are all foreign, but the presence of [the, into, a, & ~ed
] are all it took to make the sentences unmistakably English.