Why, though? 🤔 English is already a giant mishmash of tons of languages and that didn't stop us. What is "English lexicon"? Thousands of English words are either loaned or invented/adapted from Latin, Greek and dozens of other languages. Diglossia and code-switching are fun, n'est-ce pas?
Or are you just worried about Māori getting diluted and threatened by too many English/foreign loanwords? Then I probably agree with you to an extent.