phrases - Isn't it redundant to use "then" after "if"?

Since "if P, Q" is grammatical, is it not the case that the "then" in "if P, then Q" is redundant?

Where P and Q are clauses.

For example, "if it rains today, the road shall be wet tomorrow" is grammatically impeccable. (Or is it not?)

Doesn't that mean that the "then" in "if it rains today, then the road shall be wet tomorrow" is redundant? I am referring to the logic that using a word would be redundant if the same meaning is conveyed without that word.

But I see that, when the antecedent clause gets too long, the occurrence of "then" serves to mark the distinction between the two clauses.

Except in that sense, can we not say that the usage "if ... then" is redundant in English, and should be replaced by "if ..." especially if the clauses are short enough?

Ah, except in programming languages, of course.

Answer

I would not consider it redundant to have if...then...

Leaving aside the point of using then to clearly mark where the consequent clause begins, the use of both if and then can serve to emphasize the causal nature of the antecedent, or to make it seem like an if and only if rather than just an if-then.

For example:

If it rains, we will stay inside.

merely provides the plan of action in the case that it rains, whereas:

If it rains, then we will stay inside.

seems to suggest that the staying inside will only happen if it rains (note the emphasis on then, which would be stressed in speech and italicized in writing).