We should be clear that automation can reduce testing time only for certain types of tests. Automating all the tests without any plan or sequence will lead to massive scripts which are heavy maintenance, fail often and need a lot of manual intervention too. Also, in constantly evolving products automation scripts may go obsolete and need some constant checks.
I think we can all agree that automation is a critical part of any organization's software delivery pipeline, especially if you call yourself "agile." It's pretty intuitive that if you automate testing, your release cycles are going to get shorter. "So, if that's the case," you might say, "why don't we just automate everything?" There's a good reason: automation comes with a price.