I don't think De Gea is finished by any stretch, but he is perhaps a little complacent. Bringing Henderson back for next season and letting them both fight for the starting position could be good for both of them. Probably no reason to rule out Romero either, though I'd like for it to be Henderson given his age and he's an academy product, he could set us up for the next 8 to 10 years. On current form he should be England #1 too.
I'd also like to be wrong about De Gea, and whilst I don't think he's finished, his career seems to going in a Casillas-like direction. Neither were ever the complete goalkeeper but made up for that by being excellent stoppers, and that is on the wane a little bit.