A new client download and install takes about 2 minutes, it's not that big of a problem. Even so, the miners would have to either switch to use bigger block sizes after the fork happens or somehow indicate that they are supporting the bigger blocks before the fork (e.g. the supermajority fork process). This means that that larger block size should not take effect immediately.
This is indeed an issue as it could divide the network and create a lot of havoc . I personally believe XT's (and as suggested here BU) 75% threshold to be dangerously low as well. A 95% supermajority with a minimum 2 week grace period and alerts sent should be the default for hardforks. Developers have a responsibility to insure that code changes don't effect users investments. The loss of trust by the code itself losing assets would be extremely negative PR for bitcoin.
And small block adherents, imagine the reverse, if it were Mike and Gavin were running Core and Pieter, Wlad, and Maxwell had broken off and started their own implementation, with maybe Jeff going between. And people were sticking with Core and its giant block plan, heading for catastrophe. You might notice the market friction then.
One doesn't have to pick sides. I respect all of the developers above and can have nuanced opinions and disagreements with individual aspects of their code contributions.