I wonder if there's any chance of a new major architecture becoming successful today.
At best, we might see an open source contender gain some traction, but there's not really the same lineage to draw on there either. Linux at least had a few decades of Unix software and UI (including text consoles) going for it. An open source competitor to iOS or Android will either be copying what they do, or making it up as it goes along, which is unlikely to yield major advantages they haven't already taken advantage of, IMO.
The same was said for anyone versus IBM, then anyone versus Microsoft for almost 2 decades after that.
Each time a new challenge was able to enter and stay in the fray, it was by capitalizing on a paradigm shift; mainframes -> home computers -> smartphones..
I think the next frontier may be ubiquitous, always-on AR/VR. If a new company makes that happen before Apple or Google do, they might have a chance.
If we got some dark horse contender that wasn't taken as serious for long enough to let it gain an advantage and/or was established or large enough that there wouldn't be a lot of pressure to sell, maybe. Nintendo/Sony/Microsoft are the current contenders for that in my mind based on that criteria (with Microsoft actually aggressively pursuing it). Any other company that makes a good showing is likely going to be someone else's next meal.
Post-Jobs it's doing fine.