Let's see I could see devolution happening. Ash gets a call from juniper or rowan since his research was about evolution and she tells him of an experimental devoluter. Soon after Osshawott evolves. However as a Dewott he had to live with less advertising revenue and thus could no longer afford some of the essentials such as gold toilet paper and prime pokemon food. Also his fans dissapeared since they found him ugly. To compound the pain Piplup and Meloetta got together. Normally she had problems choosing but when Oshawott evolved all bets were off. Now Oshawott was heartbroken and his battle performance suffered from it.
Ash did not like using Evolved water types. Dewott was too sad to battle. He for the first time in living memory since roxie was dumped with the other reserves. Many of them hated Dewott because they could all remember being snubbed by Ash for oshawott in the past. This sends him down a crippling path of depression. Finally one night Oshawott decided to just end it all and slash his wrists with a scalchop. Thankfully Ash's other reserves saw and Leavanny string shot bandaged his wounds and Unfezant airlifted him to Junipers. Juniper realized how depressed dewott was and offered to devolve him with Ash's permission.
Since Ash missed having an unevolved water on his team he said yes. The devoluter powered up and then devolved Oshawott back into his original cute form. Everyone was happy but Ash's reserves. Especially palpitoad.
In other words. No way to devolution.