Is a professor who's never had to write a grant application disadvantaged in the job market?
No, this is not a concern.

Typically individuals who receive endowed professorships already have a long record of applying for grants.  You could, as the donor, attempt to negotiate some other situation if you wished.
Typically endowed professors only leave their job due to retirement or death.  If you have really endowed the professorship with enough money, there will be no desire to leave for another endowed professorship, if one could be found.  An endowed professorship does not make the holder uncompetitive on the job market.  It makes employers uncompetative to hire the professor.
Most endowed professors do apply for grants.

Assuming you really have that much money to give, my advice would be: Require the holder of the professorship to refrain from applying for grants.  This will free up a lot of their time to do something more productive.
If you want to make a positive impact on academia, you can achieve more for your dollar by endowing (or just spending all the money immediately on) scholarships at institutions that charge low tuition, like community colleges.