It's not as simply as an increase to SLC's coffers of 18.33 million, if they city were to annex the Millcreek area.
Salt Lake City has to take over street maintenance, including plowing, repair, street lighting, and eventually change all the street signs. They would also need to take over fire protection, police protection and garbage collection. Those things alone would require a substantial increase in expenditures for personnel and equipment.
Those of school age then become part of Salt Lake City School District, requiring the city to purchase the Granite District Buildings in the newly annexed area, to make them part of the SLC School District, and educate those children. Libraries would need to be accounted for as well. The city has it's own library system, requiring them to either purchase an existing county library or build a new one.
Other things that have to be taken into consideration is additional staff, in most departments to service/manage those areas; such as larger planning staff, and two additional council members (or the current council district would have to be enlarged dramatically.
Parks are another consideration. Salt Lake City owns it's own parks, it's rec centers are operated by the county, but parks are not. That again is additional equipment and personnel that need to be purchased and hired to maintain the additional parks.
Of course Holladay wants the area, because they currently are part of Granite District, SL Co parks, SL Co garbage and SL Co roads. So they just extend their contracted area rather then add facilities, equipment and personnel.