I trust linguists and speech and informal writing over formal writing. I have seen some terrible formal writing, although some forms which look/sound ungrammatical now were grammatical at one time. (Such as persons instead of people, ability/opportunity of +pp instead of ability/opportunity to +inf.)
and some other forms look/sound ungrammatical now because they were ungrammatical all along. (Such as adverb to verb instead of to adverb verb or to verb adverb, or shall and shan’t instead of will and won’t for the unmarked future. Shall/Should implies obligation and related concepts. Will/Would can imply intention, but can also mark tenses and situations.)
