You have to wonder at what point it will become stale, or they run out of task ideas or suitable guests. Linked to that, I question if they should leave it longer between series' as they're only having 2-3 months breaks now.
The format can run as long as they can deliver with guests I think, and there's still a long list of UK/Irish comedians who've not done it yet and would be great. Jennifer Saunders, Bill Bailey, Adam Buxton, Chris Addison, Josie Long, Richard Ayoade, Marcus Brigstocke, Rob Brydon, Robert Webb, Jack Dee, Rufus Hound, Lee Mack, Milton Jones, Miles Jupp, Dianne Morgan, Lucy Beaumont, Ross Noble, Mo Gilligan, Russel Kane, John Robins, Issy Suttie, Matt Lucas...
I mean it's a pretty endless list really. I can't see them ever running out of willing, funny people.
It's almost a blank canvas because the tasks allow a huge range of creativity and the contestants all bring something different. The element of competition keeps it interesting even though it doesn't really matter who wins.
Frankly I hope they get to 25 series so that they can do a Champion of Champion of Champions. Will already have three of the contestants in the next 12 months or so. That person will have won Taskmaster