A little late to this discussion but I'd like to add some of my anecdotal evidence/experience to this.
Tldr: the truth on the ground is more convoluted than simple articles can tell. While the ticket was a success and the research bears truth, the criticisms are not unfounded.
I moved to Munich a month ago and so, as a foreigner, have had an interesting interaction with this system.
The main thing that gets missed in every article is how convoluted the whole transit system is. In Munich, there are 2 providers, overseen by an umbrella cooperative made up of these 2 companies. There is also a zonal system that dictates how much you'll pay for the ride, and every state then has some version of this system.
As you get out of the metro and into regional train travel, there are also various classes of trains (run by a single provider nationwide). For example regional trains vs regional express trains plus different classes in each plus the ability to reserve seats vs not. There is not a single monthly pass, but a variable one that depends on which zones you'll travel on. Of for one day you have to go to a zone not covered (such as a satellite town of Munich, you'll have to buy a ticket)
This makes getting a ticket expensive but also confusing.
It's confusing and expensive enough that it's sometimes easier just to rent a car.
Now Germany is a car producing nation, and car rentals here are extremely cheap, with car sharing startups funded and subsidised by almost every German auto manufacturer.
In Munich at least, cycling is also quite popular(why specifically is something I'm not knowledgeable enough to speculate).
What the 9 euro ticket did was not only make rail travel cheaper than other options BUT ALSO significantly easier to navigate in terms of ticketing.
(This argument may be refuted by some as they argue no one checks ticket purchases on the metro trains and so incentives to purchase wasn't really a thing within the metro. Not sure how common checking was before the 9 euro ticket but we were never once checked while taking trains in the metro area but almost always got checked when taking regional trains).
From my experience, people in general who don't regularly commute and tend to drive when they do still have a close to normal tendency to drive.
It's also not easy or economical to park within the city, so most people who live and commute within the city wouldn't have been driving in the first place.
That said, it changed the calculus for us(and likely for people who are driving neutral). We would have preferably rented a car to travel around, mainly because we can, but also because it's cheaper and more convenient for longer distances. We've travelled to other states and even to Salzburg in Austria with the train with this 9 euro ticket. We've probably saved 60 euro on a monthly metro ticket and hundreds for regional travel.
There is a cost to this value however. We had to give up time in return, as taking regional trains requires multiple train hops as well as taking slower regional trains.
Also, express regional trains are not included in this 9 euro pass
And so this is where the narrative deviates or muddies at the least.
Firstly, to reinforce the primary point. The ticket has proven to be extremely popular, basically more than anyone expected.
As the research proves, it has changed the calculus for a large subset of people on what mode of transport to take. And the reasons for this includes both cost and ease.
But commuting using public transport does require a time commitment, and so reducing price and complexity likely maimly chips away at the margins.
It would be easier to see this as elasticity in the demand curve. A 9 euro ticket is extremely cheap and extremely easy to grok mentally, hence pushing demand up.
It's also removed all complexity in dealing with tickets in each state, so together with price, had driven up demand to travel interstate.
Unfortunately, the demand was significantly higher than capacity. Regional trains in Germany are not built for capacity and the system has at many times been completely overwhelmed.
This likely has affected people who take cheap regional trains to travel between towns. It however has not affected business travel or people who already pay for first class or express regional.
It has made more people travel but the argument that more people are travelling and thus producing more CO2 are wrong because marginal cost for CO2 for a train is close to 0. Only when they add more trains does this calculus change. They have likely had to do so for more popular routes.
Germany has many towns that are dependent on tourism, and so this has likely increased commerce in these towns. This is a net positive for Germany, as domestic tourism is almost always a net positive to the economy.
In summary, the 9 euro ticket was a huge success, but it's not sustainable for a primary reason: the train network is not able to support that demand.
I personally would not support a euro ticket at 9 euros. I would definitely urge the authorities introduce one that is pricier but more sustainable (and also eliminates these complexity).
The benefits accrue beyond just savings on price: reduction of inflationary pressures on people, reduced complexity, reduced emissions and increased domestic tourism.