Long distance trains every 20 minutes are an important part of more conveniece, if ticket prices are not higher if passengers choose the train that just suits best to their arrival time at the station.
Trains every 20 (or 10) minutes also allow for a better service at mid-sized towns along the route, which might be served once per hour: Then only 1/3 of the trains would need to stop at such towns, meaning that trains can achieve higher speed, serve more stops or a combiniation of both.

For explanation: More frequent trains only increase conveniece if passengers can usually use the next train available, when they arrive at the station. When changing trains, a missed train would be less relevant if the next train departs just 20 minues later and the ticket can still be used there. Some people arrive 25 minutes early at the station just to be sure to arrive in time; they could use a train 20 minutes earlier if it exists and the ticket is accepted.

More frequent trains on a main axis open more opportunities for direct trains to towns beyond. E.g. if there were trains every 10 minutes from Hamburg to Berlin, one of the six hourly trains could continue to Frankfurt (Oder), one to Cottbus, one to Dresden, two to Leipzig and one to Halle (and always beyond). Further trains to Halle and beyond to Munich would start in Berlin or at other places.

I believe the much longer number of railcars in long-distance trains (up to 13) compared to regional trains (often 4-6) is a mayor obstacle to more direct lines, as the capacity of the long trains is not required for the regional connections. But the length is currently required for recovering the track access fees.
Also, trains should not run half-empty at periods of the day or the year with low demand. Shorter trains at such time might be more important than reduced timetables.

Real life observation of public transport proves Mr Schwämmlein wrong.
Half hour interval have proven successful in a lot of european countries , nation wide or on part of the networks , for instance on several high speed lines and not least in Germany where part of the ICE/IC network has an approximate 30 min headway. this to such an extent that 15 min headways are being considered on some networks.Coming from the coach industry , he knows frequency is part of the success of manycoach operator

As for the 80% capacity reserve it must be a joke or does he also want to digitize station platforms ? Capacity at main stations is limited and so is capacity at main junctions. Successfull Capacity enhancement projects like Thameslink take decades to complete , mobilizing a lot of ressources for a very small though very important parts of a network.

Another good example is LGV Paris Sud-Est where headways are of 6 min . 80 % more would bring that to little more then 1 min ,but station stops are longer then that ! And stations like Gare de Lyon or Lyon Part-Dieu are already streched beyond capacity – actually people have to be retained off the platforms prior to boarding . How do you solve that by digitizing?

The LGV example seems very localized but actually it costrains capacity on all the Sud Est réseau : even if further south there would be capacity available , these trains need to be able to get north. You can find a lot of examples like this all over Europe .

The 80% assumed easy increase in capacity is for Germany, not France or the main axis Paris-Lyon in France.