I think he should be dropped altogether if he is on the wane. He is in danger of becoming another Ian Bell: a fading titan who can only score zero-pressure runs in the sunshine. That just isn't good enough to justify the role of opening bat for England. I don't think it has come to that yet (and the guy is only 32), but I think he should take a look at himself and recognise that he has to take the responsibility of being the senior batsman in the side. Admittedly, I think he would be helped enormously by a solid No.2, who could rotate the strike and ease the pressure. It's not yet clear whether Jennings can do that consistently.
I'm afraid to say I am just as worried about Jimmy Anderson. He's a legend in English (and, far more importantly, Lancashire) cricket, but he is bowling less and less outside tests and putting less and less pressure on opposition batsmen inside tests. Clearly his pace has dropped as he's got older, which is inevitable, and now he is more or less a pure swing specialist. Therefore he can still pick up five-fers when the conditions are right. But he looks toothless against set batsmen on true pitches in clear conditions. And what are we going to get in the Ashes in Australia? Pitches like a bowling ally under blazing sunshine. I think he will wilt, and I think Chris Woakes is going to be able to do Jimmy's thing better than Jimmy in those conditions. The team may well be better served by an additional specialist spinner, an express fast bowler, or another useful all rounder taking the spot. Mason Crane, either of the Overton brothers, the ever-useful Liam Plunkett, or even an old hand like Tim Bresnan (who averages 48 with the bat and 31 with ball in first class cricket) are just a few contenders that could be useful.
I should make clear that I hate saying all this, and I hope I will be proven wrong. But there shouldn't be any sacred cows - it was the refusal to consider dropping the old guard that led to England losing 5-0 in Australia and then a test series at home to Bangladesh before having to rebuild almost completely.