There's no doubt that the subjunctive in English has lost much of its historical scope. There used to be present and past subjunctive for all verbs, but the only vestige of the past subjunctive is the irrealis use of were that I referred to yesterday. When you've only got one form, it's hardly worth calling it a system. The simple past tense has taken over much of the work in sentences like If you saw it, you'd know.
Many claim that the subjunctive continues its fade today, but there are a brave few, such as Charles Finney of the University of Tennessee, who actually argue that it's making a comeback. I'm very pleased to see that Finney is at least somewhat interested in the data. He keeps a list of examples that he runs across to illustrate his point. The problem is that you can't show a revival by looking at a single point in time.
So to get a time series, I went to the TIME Corpus and the COCA and used that [pronoun] be as my stand-in for the whole of the subjunctive. I'd say it's a reasonably representative sample, though clearly far from perfect. If you're playing along at home, the query would look like this:
The result from the TIME corpus shows a clear drop off over the last century with only the 1930s and 1990s going slightly against trend. The most recent decade has, however, been very hard on the subjunctive:
The drop off in the last two decades is confirmed by the COCA, though it doesn't look quite as desperate:
The thing I found most interesting is that spoken English appears to be the biggest user of the subjunctive, though academic English isn't far behind.
So it appears that the subjunctive really is continuing its decline.