Our wisteria is dying - cause, and how to replace?

Our beloved 12 year old wisteria appears to be rapidly dying. Over the space of a week, all the leaves have started shriveling, turning crispy and dropping off. However, it had shown signs of being unwell for months.

Pictures showing shriveled leaves, base (ignore toys), and canopy (all leaves shriveled):


For years I've been aware that the vine had a curiously bulbous base, much wider than the main vine, with some parts of it appearing crumbly or hollow. Even though it flowers beautifully, both last year and this year I was concerned by its lack of vigour, putting out surprisingly little new growth. And a couple of weeks ago we first started suspecting the leaves seemed dry-ish, not what you'd expect for mid-summer.

All the leaves on the plant have been shriveling at the same rate, without any markings that suggest insect damage or mildew. It's not growing in a container, and the garden's soil is dry rather than damp.

From my online research, the cause seems to be either root rot or graft failure, but I'd appreciate a second opinion. If it is root rot (and how do I tell?), what should I do if I plant a new wisteria in that location, without it succumbing to the same thing?

Bonus question: what is the quickest way for me to regrow a green canopy over our trellis next summer while waiting for a new wisteria to grow? I was thinking some fast-growing creeper in a large pot -- morning glory maybe? Or maybe just take a cutting from our golden hop which grows on the other side of the trellis.

Wisteria in happier times:

 

Moisture. Watering. Get in there and cut away all sickly whimp growth. Don't feed.