Bumping the price per time decrease is always an option for them. The real hard core spenders will keep spending no matter what to get the edge. Though that route may make it a lot more difficult for light spenders to use the time feature.
If they instead choose to use 99% as the maximum reduction, the last wall level still takes 5 minutes to complete. That's much cooler than 10 seconds still, and would not modify the PP cost as instead of using the instant complete at 10, they'd use another half to get it down to 5 minutes.
So the poll ends tomorrow. Tribal Wars developers, please consider this with a 99% maximum reduction. It should have the same pp cost or more because instant complete wont be available and so extra PP is spent halving past 10 minutes left.
Also, maybe it can be calculated simpler such that the last possible time reduction just sets it at 99% of the time, even if halve would've caused it to reduce more.