"While the cost of resistance is highly variable, such resistance mutations or genes often come with a fitness cost that reduces the rate of bacterial proliferation (Dahlberg & Chao, 2003; Melnyk et al, 2015)." https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5371735/
"The success of resistant mutants critically depends on rapid counterbalancing of the decreased fitness by acquiring compensatory mutations (Levin et al, 1997; Marciano et al, 2007), which in most cases restore normal growth while preserving resistance to the antibiotics (Marcusson et al, 2009). The number and variety of compensatory mutations required to successfully compensate fitness cost varies with organism (Palmer & Kishony, 2013, 2014; Cheng et al, 2014) and the particular environmental conditions under which compensation occurs (Testerman et al, 2006; Hoffman et al, 2010; Toprak et al, 2012; Lindsey et al, 2013)."
Basically, resistance become extremely important so anything that works get's adapted. But, after that point the search for mutations that reduce cost shows up. The second stage also depends on continuous exposure to antibiotics as reverting the mutation in a population is relatively simple.
Antibiotic resistant bacteria do kill people, but most often as secondary infections due to a compromised immune system. Further, even if the primary infection has near total resistance suppressing secondary infections is still extremely useful.