It's absolutely true that you can't know anything for sure about the schedule until the games are happening. But when this gets (correctly) used as a response to people who worry about a schedule in April, it (like all other stances) inevitably goes further than it logically should, because you absolutely CAN have SOME idea for how tough a schedule can be beforehand. The board consensus about this year's schedule began before last season was over, and was largely correct. Because while some teams are surprises, MOST teams follow typical positive or negative developmental paths, give or take a couple of wins. Which is why we have concepts of rebuilds and declining contenders in our heads in the first place.
Even if the teams were bad, lining up two western divisions is tough for travel purposes, it is logical and likely statistically demonstrable that cross country games are harder than ones nearby, all else equal. Adding the fact that the Seahawks, 49ers, Chiefs don't look to be falling off any time soon, and most of the other teams aren't jokes, and the fact that we'll have a 2nd place schedule, it's completely reasonable to suggest that we'll have a tougher road next year, especially compared to this one.