A Forbes article -- apparently written by an American -- titled "How To Tell If A Wine Will Age" has this passage:
The other structural elements of body, alcohol, and sugar can also point to an age-worthy wine. Full-bodied wines, which also tend to be high in alcohol, are more likely to age well, but it’s important that all of that body is supported by acid, tannins, or both. That rich, soft $10 red purchased at the supermarket is easy-drinking now, but without firm tannins or bright acidity, it will not improve at all over time. Sugar can also help a wine age, though the perception of sweetness will diminish through the years.
Assuming this is American English, is there any reason why the verb in the that-clause is not "be" but "is"?
I thought it was only informal BE that would allow "is" in this type of clause.
Here, the writer is saying that some cheap reds may lack in tannin or acidity, in which case the strong flavor of such cheap wines is going to diminish over time.
So, I don't think the writer is making a factual statement in the bold clause, but rather is presenting a condition under which full-bodied wines can age well without losing the full-bodiedness over time.