It's easy to look back on X (2013-17 version) with some rose tinted specs, but it should be remembered that even that version wasn't that popular.
The ride suffered when Storm Surge was introduced. It has become hidden away and off the beaten track. You could forgive guests for forgetting its existence.
X was a fun experience with it being a 'roller coaster rave'. But the wider audience didn't know what it was or what to expect. And the roller coaster rave idea on a 1m height restriction ride causes confusion - is it a kids-family ride? A ride for everyone? A ride for the thrillseeker? No one really knew.
The Walking Dead overlay solves some of the problems X experienced. Moving the entrance so it was more prominent was smart. It has a clear and defined audience. Those are problems they could have solved whilst keeping it as X, but it at least solved them. The throughput has probably improved since the overlay too, which is good given X struggled. And tbh, I think it's a pretty solid experience on the whole - Thorpe's answer to Van Helsing's Factory if you will.
So what's the problem?
1. The ride has a defined entrance, but is at a dead end path. And the ride is still hidden by Storm Surge. People still struggle to find it.
2. The park is saturated with scary experiences. You have Saw and Swarm offering dark / apocalyptic themes. And of course the likes of Ghost Train and now Black Mirror have bleak stories (although the latter in a very different fashion). Even if this is what the wider audience reacts to, people will become fatigued with this.
2a. Related to this, the ride probably has a reputation of being like a 'year round Fright Nights attraction', in one way or another. Whether people confuse it for the mazes they had, if that's the reputation it's got or if that's what the park have tried to convey, that's probably the reaction it's getting. And simply put, that doesn't work for Thorpe outside of Halloween. Look at any 'proper' scare attraction they've tried doing outside of Halloween season: it's failed miserably.
3. The mis-match of theme and ride. You have a very dark theme and a family ride. Stuff like that can work (I'd argue that again Van Helsing's Factory shows that), but for whatever reason it doesn't work here.
The ride is a weird one in terms of popularity - it either seems to attract a short wait time, or a very long wait time. Why that is is anyone's guess. I wonder how much the current 'even numbered groups only' restriction is affecting the ride's popularity (I'd assume not much, but more just a musing I'm throwing out there). But I don't think it's necessarily an unpopular ride. It's just a ride and overall experience that doesn't quite work for the wider context of the park.
So, the question: is it on borrowed time? I'd say no. It's ticking along fine for what should ultimately be viewed as a filler ride. When the Walking Dead license runs out (and when the park choose to not renew it), they can very easily drop the Walking Dead name and turn it into a generic 'escape the zombies' ride if they wanted. It doesn't solve any of the problems, but it can exist quite easily in that state.
The issue with the attraction is the complexity in scrapping it. Removing the ride, but keeping the building itself, is costly (I've heard upwards of £1m). Removing the actual building as well as the ride would no doubt add to the cost. If you try to put something else in that building, you have a pretty awkward scenario to work with, to the point it would make more sense to scrap the whole thing.
Obviously they could retheme it again, but with Storm Surge in existence, it's hard to see how any retheme would really help draw attention to the ride long term.
A dream world scenario would see the Storm Surge, pyramid and Slammer site flattened, and have that section used for a couple of rides (a log flume and coaster intertwined, for example). Or if they ever removed Colossus, remove the pyramid then and do something similar.
Heck, even just the removal (or relocation) of Storm Surge would play a big part in improving the ride's popularity and worth.
In short, the Walking Dead ride ticks along fine imo. It's popularity suffers due to the location and saturation of horror experiences at the park. Any retheme will likely only be a short term fix. Scrapping it is costly and only really makes sense as part of a huge redevelopment of the surrounding area.