Sforno on Numbers 3:4/ספורנו על במדבר ג׳:ד׳

1בהקריבם אש זרה, this was the only blemish they had been guilty of.

2ובנים לא היו להם, if they had had sons, the sons would have been their replacements if they had been physically and mentally suitable.

3ויכהן אלעזר ואיתמר, they had been given authority to substitute for their father in the Tabernacle if the need would arise, by the command of G’d.

4על פני אהרן אביהם, during the lifetime of Aaron his sons had already been anointed. The death of two of them, as well as their replacing their father as High Priest was not at Aaron’s command, even though he was the High Priest. [if I understand the author correctly, he means that contrary to a king appointing a son as his successor, the High Priest is not entitled to appoint his own successor. Ed.] None of what is reported here is of significance for future generations as ordinary priests were not anointed, whereas the position of High Priest was not a position that could be inherited.