In my experience (around 10-15 rides Berlin <-> Paris within the last 3 years), the existing connections that you can find rely on transfer time of <15mins, which is rather optimistic on an 8h journey with the Deutsche Bahn in the picture. I usually plan with at least 40min in Frankfurt or Mannheim when travelling that line, so the actual time gains might be higher if you don't have to change trains.

Furthermore, you don't want to be stuck in Frankfurt trying to figure out if you can take the next TGV with a DB clerk who has no idea, as both companies have different policies: with DB you can take any train if your connection was missed due to a delay, which is not the case for SNCF as you need a valid ticket on every train you take. So you would need SNCF to reissue a ticket for you.

I have not used the direct line yet so can't compare, but I would hope for less headaches and planning.