In a sane world, taking a meeting in which nothing of substance was offered and no money changed hands would be less serious than paying myriad unidentified Russian intelligence sources to lie about your political opponent and enable your own government to spy on said opponent.
Apparently we do not live in a sane world.
If the Dems won the House, Mueller was always going to come up with something to keep threatening Trump and avoid the full declassification that would expect the FBI, DOJ, CIA, etc to full sunlight.
I guess this is it. Mueller can effectively threaten Trump Jr with prosecution for that meeting and keep the threat live for another two years while Trump has to run for re-election.