And what happens when your system scales out to 2 hosts? Now you have 2 txt files - no big deal just query them both. Now what about when you scale to 16 hosts? 100 hosts? What about when your system is composed of dozens of services all managed by different teams with different logging philosophies? And hosts are created and destroyed dynamically, where should the txt files go?

Tools like New Relic are not designed for small products that can run on a single host with logs in a txt file. They provide value when systems scale.