Will downcountry bikes make trailbikes obsolete? I doubt it.
Pennyfarthings are obsolete, for good reason as they are dangerous. They are ridden only by a few quirky folks. Are single speeds obsolete? Arguably the deraullier is the greatest advance in cycling since the “safety bike” was developed yet still single speeds are around. I live on the great divide mt bike route and am surprised at how many people are doing the route on them. I understand they are gaining in popularity as commuter bikes.
The industry now has road bikes, touring bikes, adventure or gravel bikes, track bikes, single speed commuter and mountain bikes, hardtails, enduro bikes, downhill bikes, xc bikes, cyclocross bikes, trailbikes and now downcountry bikes. Give it a few years and there will undoubtedly be more tweaks with new names.
I recently got a new trail bike because I was feeling sketchy on some routes on my hardtail with no dropper seat, and at my age have to to avoid spills, and I thought the dual suspension bike would be more fun (it is) and possibly safer. I suspect it is the last bike I will purchase. Dropping another few thousand or a few lbs in weight, or some other advance which might be nice but which I do not really need, is something I don’t think I could justify. If it means avoiding certain kinds of trails – well, I do that already.
Many people are “bike nerds” and drool over the technical aspects and specs. More power to you, the industry needs you. Many, I suspect most, just want a bike that will do the job – and trailbikes do that well, in addition to being like swiss army knives in that they that will handle just about anything. I suspect they will be around a while. Just my opinion and I am not a bike nerd so I may well be wrong.
The one recent bike development that has indeed been a major game changer in my area (Montana) has been fat bikes. I see people riding them up the south hills trails near Helena in the depths of winter, when other bikes would get bogged down immediately in the snow.