The solid fuel was not a mistake, but sulfur fits the new paradigm of direct-to-gas better by not forcing use of the inefficient gas->solid recipe prior to other oil fractions becoming available.
It also crafts much faster and requires less gas than solid fuel.
Finally, it will inspire the distribution of solid sulfur to more places than before, with creative benefits. Solid fuel already had a role in extensive fuel distribution systems, along with being an ingredient in rocket fuel.