Is it possible to deactivate the "New Learning Experience" per Default?
Yes it is, and there are a couple of options. These are both managed in Learning Administration > System Administration > Application Administration > Integrated Learning Settings.
If you want to keep the New Learning Experience available, but just have it defaulted to the old version, select Off for the setting "New Learning Experience swith default setting".
If you want to disable the New Learning Experience entirely, you can simply tick the "Disable New Learning Experience" checkbox, and then select one of the reasons.
If you then want to enable the New Learning Experience at a later date, it's just a case of unticking this box and it becomes available again straight away.
I hope that helps!