yeah but that would also be the case for lv 38 55 or whatever. You can duplicate everything not only lv 70 +15. And also for game breaking bugs isn't the pob here because of that? Was there not already enough time to find bugs? Why not extend pob if you think that there are more game breaking bugs? A lvl cap won't help Daneos or the community to prevent bugs / bug abuses.
You aren't thinking enough on it really. Now back to your point, the bugs did happen and they can happen again but it actually gives Daneos time to work on quests and fix all the stuff he has to. It will actually help him prevent bugs like duplication and stuff like that and people are already tired of PoB.
Benefits of playing in a different cap: In level 55 cap, SKs and SMs and other classes who had a better PvP chances will actually do very good there and give players new experience. People will start doing other dungeons such as UD4 (who are forgotten really lol) and spam a lot of TMQs. It is all about giving players new experiences that they probably wouldn't even try doing and actually give them time to work on maybe new level cap? Fixing quests that are broken? Now, I have a question for you, would you rather have lvl 55 +15 bugged or lvl 70 +15 bugged? Maybe he actually even fixes the broken PvP classes (looking at you Karmas and SKs) and fix the classes that nobody uses (DW, Chef).