The project has been consistent in saying that they want to support future Pixel models, so I think it's safe to assume that they will do so if they can with reasonable effort. Google could do something dramatic like permanently lock the bootloaders on new Pixels (although I think that's unlikely), but the more likely problem would be if they just did something that made support way harder, and the team ended up having to make a pragmatic decision to focus on, e.g. Motorola who is cooperating with them vs. expending resources on Pixels. A version of this happened when Google stopped publishing device trees. That probably wasn't intentionally meant to hurt GOS; it was more of them just not caring. The team ended up improving their adev tool so it wasn't a huge deal, but something similar in the future could lead the team to ultimately decide it's not worth it.