I think you're overestimating the influence fans have here. Thorpe (and any park really) aren't going to rush a big new investment because fans (which make up less than 1% of visitors) are "pressuring" them. In this sense, fan opinion really means nothing: they'll add a new coaster when they want to.
They won't do that until they replace them. Sad fact, but true.
tbh, Loggers doesn't even need removing for the sake of removing really. You can hardly see these days. I'd rather them invest the time, money and effort into something else. And anyways, the only thing I can see replacing Loggers is a roller coaster, since they wouldn't be able to open a new log flume there.
Slammer is easier to replace, but since Merlin rarely seem to invest in flat rides - and there's a very small list of suitable flat rides for Thorpe - it's hard to see what they could replace that plot with. I think it's more realistic to expect a coaster to go there and in the surrounding area than a new flat ride, for example.
So really, the bigger issue here is Merlin's "risk free" strategy. They won't invest unless they feel basically certain they can make some money from said investment.
If this is true, then just..wow.
Wickerman bought a noticeable increase in numbers. The brand is reasonably strong and I've seen quite a few people with related-merch. It's a slow burner of an attraction too; it had the instant hit, and it'll also have to repeat value of people coming back to it in years to come.
If everything that Wickerman has done for the park hasn't led Merlin to say it's been a financial success, then maybe, just maybe, they're setting the bar too high.