That problem already existed even without the hopper ticket. Firstly, if your journey involved taking a bus, then getting a train or tube shortly before peak fares kick in, a delay to the bus journey could result in your paying peak fares for the subsequent rail journey.
Secondly, if travelling late at night on multiple buses, a delay to the first bus could result in the next bus being timed as next-day, which would result in higher fares if you were close to the daily cap.
I'll grant that those (especially the 2nd one) are likely to be less common than someone using multiple bus journeys, but the fact that you don't ever hear about complaints about people paying more in those situations rather suggests to me that the hopper ticket will work fine in that regard too.
It also crosses my mind that, if someone really is so close to the hour on their first journey that a delay pushes them over the 70 minutes that appears to be the actual window, then they were probably making quite a long journey in the first place - and therefore possibly not within the target market for the hopper fare (although that in itself probably won't stop people complaining if they feel like it).