Don't we have sensor or laser technology that can replace them? And they would be more accurate. I mean my goodness there need to be 10 line judges per court. What a waste.
I sort of questioned that myself a while back, but I feel the service judge needs more attention, so much inconsistencies there.
Some major tennis tournaments have implemented real-time Hawkeye and do away with line judges. I'm guess such thing will come for Super 1000 in a couple years time. No more challenges, or rather, having players think about running out of challenges. That'd be nice.