It's very likely that you'll see Google Assistant on new Android devices beyond the Pixel line, but when... and what about the device you already have? The AI helper might be coming sooner than you think. Google has inadvertently given beta testers an alpha release of its Android search app that enables Assistant support on non-Pixel hardware. It's not working for everyone, but people carrying everything from the Nexus 6P to Alcatel's Idol 4S say they've had success. The software has introductory messages that are clearly targeted at people who previously didn't have Assistant, so it's not an accidental inclusion.
The alpha also includes a recent topics section that revisits recent searches, previously mentioned support for payments (not that it's useful without providers) and a data-saving Lite mode that optimizes pages.
It's going to be a while before you see an official rollout when the app isn't even in beta. You may have to wait weeks or months before there's a version you can reliably use. However, the very fact that wider Assistant support is present at all is important -- it shows that Google wants to broaden access fairly quickly.