For the sake of argument, I'll pretend all of the extremely strong points like 'Johann is blind' (which does not, by any means, prevent someone from creating art, but is an undeniable handicap when trying to perfectly replicate a vision) and 'Johann is not an artist' and 'the source of Johann's insight is a shitstorm waiting to happen' don't exist. Even with all of that aside, Johann being alive now does not meaningfully improve art he creates, compared to art created by those who were alive when the Ancestor Gods were alive. They had decades or centuries to observe and get to know the Ancestor Gods to better depict them, Johann had a fraction of a second. The only potentially positive quality a theoretical art piece that Johann creates would have over all of the existing art is novelty, and Dwarves generally aren't super big on novelty.
Dwarves wishing to commune with the achievement of gromril can walk the shafts that the Ancestor Gods walked and touch the stone that they worked. This is always going to be more meaningful to them than some theoretical sculpture or painting or whatever created by a Wizard who got a glimpse of the moment through Umgi trickery.