Is it legal for the elevator company to dispatch all on-site maintenance personnel with labor services?

Illegal. According to relevant data, it is illegal to use dispatched workers in large quantities, and there are restrictions on the use of dispatched workers by employers. Article 4 of the Interim Provisions on Labor Dispatch stipulates that the employing unit shall strictly control the number of dispatched workers, and the number of dispatched workers used shall not exceed 10% of its total employment.