We don't need modular batteries, we need accessible cells. Train mechanics to find and replace the bad cell. Spot welding a new cell in is a skill, but one mechanics can learn. There are safety concerns with this, but high voltage spark plugs are not safe either and mechanics work with them all the time. However if the cells are not reasonably, or they can't find the bad one the whole car becomes scrap.

Some of this will come as mechanics decide they need to learn how to service EVs. However some of it depends on a good design for repair.