I'm kind of wondering if they suddenly realized that they've painted themselves into a corner. By removing all the flat rides that weren't considered crowd-pullers on their own, and focusing on adding new "headliner" attractions with a strong branding, they've found the park suddenly lacking a "backbone lineup" of thrill attractions. Yes, the park has a fair few coasters, but aside from those there's just Duel, Hex, two water rides whose appeal is very weather-dependent, and a couple of aging flat rides that are probably due for replacement very soon (being around 40 years old). There's not much to spread out guests away from the main coasters, little to fall back on if something breaks down, and queues congregate mainly at a handful of rides. Despite its size, Alton Towers appears to have very few "filler" attractions that aren't strictly kiddie rides.
Or in other words, "Oh crap, we've torn out half our park and now there's little left to keep guests inside the gates". The addition of leased rides could be a measure to try to patch up a lack of thrill rides after so many of them have left the park over the past eight years.
It probably looks like a good decision in the short term, to reduce maintenance costs by retiring an old ride. But if it's done again and again, there's not much left of the park. Maybe they assumed that a new coaster would be several times as marketable, so that Wicker Man could serve as a replacement for a handful of lost flats? Perhaps that's true quality-wise, but the quantity has suffered. And it's going to be expensive to build that back. Maybe leasing is the only viable way for the park to rebuild its total lineup in the short term.
Thorpe appears to be wading into those same waters too, with a whole slew of attractions due for replacement in the 2010s that just never happened. Trying to catch up with that backlog will cost them a lot of money as well. But at least they're not Six Flags cirka 2030, when they will receive that same problem, times a hundred, across the whole chain, when literally dozens of huge coasters will reach their "replace by" date every year.