This market supposes that after 25 series there will be a Champion of Champions of Champions special, which would mark a significant numerical moment that wouldn't really ever be matched. So they might decide to wrap it up there.

There is little chance that they slow the pace of release enough to not make it to 26 within the time frame

https://manifold.markets/Noit/will-a-taskmaster-champion-of-champ?r=Sm9zaHVhV2lsa2Vz

Will a Taskmaster UK “Champion of Champion of Champions” be aired before 2030?

75% chance. Resolves YES if a Taskmaster special airs before the resolution date which features a majority of contestants who have previously won Taskmaster UK Champion of Champions.