Grothendieck "overshadowed an entire field" firstmost because mathematicians found what he was doing important. Granted, he was able to show results, but it was not just that he managed to give some proofs, his techniques and theories (and philosophy/approach in a general sense) gave rise to entire subfields of mathematics.

In this way, it is exactly an example of what has been argued over and over: that what is important in mathematics is creating a continuity and a progress in understanding, building new theories etc.

Research output is a metric that has been chosen to guide employability, promotions etc, otherwise it is a mean to the goal of understanding, it is not a real goal in itself in building mathematics itself (except regarding careerism).