The full article with additional interviewees can be found here: Mirror.
In the interview, the bicycle professor answers the following questions:

1. What do you think of the introduction of the 9-euro ticket? 

In principle, I welcome all measures that serve to strengthen public transport. With the low fare, new people will also try public transport, which in the best case can lead to new customers even in the long term, if they have a positive experience with the use of public transport. However, I feel that the 9-Euro-Ticket is a rather short-term measure that could possibly annoy more people than introduce new customers to public transport. Especially the most loyal public transport users with longtime subscription cards or season tickets will be neglected by this measure and will not get to feel the benefits of this very cheap ticket.

2) Do you expect that the 9-Euro-Ticket will increase the attractiveness of public transport and encourage many citizens to change?

I fear that the 9-ticket will unfortunately not increase the attractiveness of public transport, as it is being implemented without any preparation by the transport associations and thus without the necessary expanded capacities. Many citizens will of course want to try the now very attractively priced public transport, but will then encounter overcrowded buses and trains. This would mean that the new customers would not be able to experience comfortable and attractive public transport, which in turn could deter them from switching to public transport in the long term.

3 At the moment, the 9-euro ticket is only planned for three months. Does this make sense, or will the control effect for traffic fizzle out?

In my opinion, the control effect can unfortunately quickly fizzle out with this measure. In addition to the problems already mentioned, citizens will keep this very low anchor price in mind and thus hardly be willing to pay higher prices in the future. Experiments with zero fares have also shown that people walking and cycling in particular will switch to the quasi-free public transport system, which would ultimately have no effect on reducing greenhouse gases. 4.

4 In your opinion, would there be a better alternative? What would make sense in order to make the switch to bus and rail attractive for as many citizens as possible, also in the long term? How would such a ticket have to be designed?

In order to encourage more citizens to switch to public transportation, longer-term preparations are needed. First of all, this includes a broad capacity expansion.  Particularly in rural areas, public transportation should be available much more frequently as a reliable alternative, but further expansion can also take place in the cities with the corresponding increase in attractiveness. Here, as in the past, subsidies will have to be provided to make it clear that the use of public transportation and the entire environmental network is the common goal. It is important that customers can easily understand the pricing system even outside their own region. At the same time, the social benefit of using public transportation should also be reflected in the price, especially when compared to private motorized transport. Overall, I envision a bundle of measures that uses pull and push measures to influence mobility behavior as a whole toward more environmentally friendly behavior. And that also means that the use of the bus, or even the bicycle, is so attractive that people don't even have to think about using the private car. 

What else would be important to you in this context?

It is very important to me to make public transportation attractive and usable for as many citizens as possible. An expansion is absolutely necessary in order to implement the mobility turnaround. The 9-euro ticket is a very short-term affair and does not do justice to this task, but it does at least send out a signal as to what is politically desired  and what would also be possible. What we really need in Germany are long-term strategies and planning options to establish true equity in the overall social context between all modes of transportation.