The interesting question to me is whether the addition of a new coaster will coincide with the removal of an old one. Given stable visitor numbers, any park will eventually come to a point where it doesn't make sense to increase the number of coasters in operation. In other words, a park can't keep growing forever. At some point, focus will shift towards sustaining and periodically renewing its lineup rather than expanding it. If the Thorpe executives feel that the park has reached this point, a new addition won't come until something old needs to be removed, and we can assume the renewal will be like-for-like. If they tear out Flying Fish, they're not going to get a B&M Giga to replace it.
None of Thorpe's major coasters in operation today are particularly old, all things considered. The Walking Dead is actually the oldest one, originally opening in 1996. That's just 23 years ago, and we can assume they expect it to last for a while longer since they just refurbished it. Colossus in second place (2002) is still not old enough to vote. True, it rides like a plastic lawnchair caught in a landslide, but its vital components may be rated for a lifetime of 25 years or more, and it might still pull in enough guests to warrant a life extension beyond that. None of the major flat rides appear to be older than 20 years either, they may yet have many years left in them.
In a sense, I can see why Thorpe has slowed down investments in recent years. It has a "complete" lineup, more or less, it has reached a size that fits its visitor numbers, and all the major attractions have several years left in their life spans. Minor additions, seasonal events and promotions could be enough to sustain the visitor numbers without building anything major over the next few years. The boom period of the early 2000s provided the park with enough rides to rely on for the foreseeable future. To go into a full cynical corporate perspective: Fans who grew up during this time might actually have become too used to this cadence of multiple large additions every year. While they see this current situation, where new additions are few and far between, as an exception; actually, the boom period was the anomaly. What we're seeing now is business as usual.
That being said: Thorpe built a lot of big rides between 2000 and 2006: Tidal Wave, Detonator, Vortex, Zodiac, Colossus, Nemesis Inferno, Quantum, Samurai, Rush and Stealth. That's literally half of its backbone lineup built within seven years. Assuming a similar operating life for most of these rides, they will also reach their functional life span in very short order too. If the Thorpe executives don't plan to do any replacements before they are needed, when that time comes, they will have to replace half the park in under a decade. The longer they choose to kick that can down the road, the harsher they will have it when a large fraction of their headliner attractions go out of order shortly after another. The investment of replacing these needs to be spread out over a longer time period, and the earlier they start, the smoother it will go.
Take the ten rides above as an example. Say that half of them can have their lifetime extended by up to eight years at the most. Say also that the other half can be replaced earlier than their life span would suggest, by up to five years. Suddenly, that 7-year period in which they reach their life span is stretched to 20 years. That's one ride to be replaced every two years. But if they let every ride run their full life span and put them all on life extension, they will not extend the replacement period at all. They'd be back to having to buy ten new rides within seven years to sustain the park's size. And those are headliner attractions, mind you. Ones that can't be functionally replaced by inflatable castles or merry-go-rounds. Even though it would cost a lot to buy each of these rides brand new, probably putting red numbers on the bottom line the year in question, anything worse than a like-for-like replacement would hurt the park and cost them attendance. The bottom line is affected either way, they will have to pick their poison. Red numbers every few years for a while, or green numbers until they suddenly go red forever. Spreading the investment would be easier on the bank, so they better get started already. Some of the rides might still have good resale value, so the net cost of replacing them could be lowered somewhat.
If I were to manage the replacement, I think a logical place to begin would be to do as Liseberg and replace Vortex with a bigger and more modern Frisbee. This giant flat ride would serve as a headliner to soften the blow of Colossus being torn down at the same time, leaving the park with one fewer coaster while a replacement is built. The Colossus replacement could be the Big New Thing for a few years (like Swarm was/currently is, depending on your opinion on DBGT) while the rest of the flat rides were replaced almost like-for-like. Nemesis Inferno could have its life extended for a few more years, it's still solid enough to be up to par with modern rides. A few years after the replacement of Colossus, Tidal Wave would be taken down and replaced (possibly with a Log Flume). Some of these replacements, most prominently the Frisbee, would be put on the island behind Stealth, finally tying those two loose ends together. The hole left by Tidal Wave would also clear up land in preparation for the day Stealth kicks the bucket. It would be the next on the list, approaching 25 years of age by this point. I'd also look into building a waterpark as a second gate at some point, removing Amity Beach and refurbishing the entrance area so the first thing greeting visitors to Thorpe wouldn't be
the backside of a waterslide.
To conclude, I don't think Thorpe Park strictly
needs any big new rides right at the moment. But it sure as heck will need several of them in the near-ish future, and it would be stupid to wait until that time comes before anything is done. My biggest worry is the possibility that they will go for the "we'll deal with that as it comes" solution, simply because it puts nice numbers on the bottom line until it suddenly doesn't anymore.