Which means she won't die unless she goes over the line and actually risks dying for potential kills. It's easy to stay alive with Tracer forever, it's hard to stay alive while actually being useful, since only Reaper has to get as close as Tracer does in order to do real damage (well, McCree too, but McCree has a stun and insta-kill mechanic, which keep him safe unless he fucks up, he also has long range on his primary fire).