My Mini 4 running on iOS 10.x feels just as snappy as my 12.9 Pro running on 11.x. Based on my experiences with iPads and iOS, the Mini 4 will remain on 10.x for the remainder of it's life.I’ve been using a mini 4 for almost a year, along with a 12.9” iPad.
I primarily use the mini for reading books, and really really enjoy its size and weight for that purpose. I do use it as an iPad in usual ways as well - messaging, browsing, etc. But compared to my 12.9” iPad and iPhone X it feels relatively slow. Painful at times even.
I don’t think the mini will ever get updated or redesigned (if/when it comes to other iPads). Hope I’m wrong but I think it’s going to slowly get phased out by Apple. I’ll be really disappointed as no other iPad feels as natural for reading books.
I agree, that the Mini probably won't get a redesign, but I think it is possible that it will get a processor upgrade in the Fall. It might be like the iPod Touch... doesn't get an update like in forever, and just when everyone collectively concludes it is dead, Apple slips out a minor spec bump. It might come as a surprise to some folk, but they still sell the iPod Touch.