If you are a miner, and you know a block of size X can be processed by 85% of the network, but not 100%, do you mine it? If by 'network', we mean hashrate, then definitely! 85% is high enough that you'll be able to build the longest chain. The miners that can't keep up will be pruned, and then the target for '85% fastest' moves - now a smaller set of miners represents 85% and you can move the block size up, pruning another set of miners.

If by 'network', you mean all nodes... today we already have nodes that can't keep up. So by necessity you are picking a subset of nodes that can keep up, and a subset that cannot. So, now you are deciding who is safe to prune. Raspi's? Probably safe. Single merchants that run their own nodes on desktop hardware? Probably safe. All desktop hardware, but none of the exchanges? Maybe not safe today. But if you've been near desktop levels for a while, and slowly driving off the slower desktops, at some point you might only be driving away 10 nodes to jump up to 'small datacenter' levels.