I actually think that 2018 was about the time when phone games had very much lost momentum and now are much less exciting than they were circa 2013. By 2018, both the potential and the limitations of phone games were very much understood by the audience. I'd argue that the top of the hype cycle of "maybe phone games will actually become really good" was 2010's Infinity Blade. Clash of Clans came out in 2012, and by 2018 phone games were fully devoid of momentum.