As a sweeping generalization, players worry too much about scaling. Back in vanilla and maybe BC, it was a huge concern. Some abilities scaled with attack power and some just did flat damage. Guess what happened when gear level increased?
Now days (unless I am forgetting something) every ability scales with attack power or spell power, meaning it scales with the primary stats that we slather on your items. If not then it at least scales with weapon damage, which is also just a function of ilevel. An ability that scales well with crit and haste and mastery will do even more damage as ilevel increases, but you have to consider the deltas we're talking about here. If your entire raid improves every single item in a tier (which doesn't always happen), your ilevel might go up by 13 ilevels. Let's be charitable and assume an ilevel is a 2% DPS increase. That means someone with awesome scaling might get a 26% DPS increase compared to someone with no scaling. But we already established all abilities scale with primary stats. So even the low end guy is going to get some scaling. Furthermore, we're typically talking about a player with one strong secondary stat and two weak secondary stats. So he gets a lot from the primary, less from the good secondary, and even less (but not zero!) from the bad secondaries.
Furthermore, specs that tend to not scale as well with their secondary stats will scale better with their primary stat. To use the warrior as an example (since that's what kicked this off), Fury scales well with secondary stats, but Arms scales well with Strength. We also think that's interesting because it makes you look at your gear and really think about the stat allocation instead of "Plate? Go." On top of all of that, you can reforge and gem to lean towards stats that are the best for you, which downplays the impact of bad scaling even more.
So again let's be charitable and say that we're talking about a max delta of something like 5-10% between someone who scales awesome and someone who scales poorly at the very end of a raid tier when everyone is completely geared, which by the way, is probably when you don't care so much about being able to kill those bosses since they're by definition all on farm. Encounter mechanics can have a much bigger impact on DPS than 5-10%.
Now this might still be a problem if we didn't make any adjustments throughout an expansion and the poor-scaling guy just fell farther and farther behind. But we don't do that any longer. Every tier these days is a new chance for us to readjust the numbers and make sure that even if there is a stat scaling issue that we can compensate for it.
Furthermore, I'm not sure why you'd assume that we check DPS only at the current ilevel. We check for ilevels that won't exist within the 5.0 time period. On occasion we have tested with stupidly theoretical item levels of 10,000 or more just to see what happens to the curves.
It also doesn't help the conversation that the word "scaling" gets misused a lot. The question above uses the term the way we would, but misuse is sadly pretty common. "Our scaling is bad" gets used as a synonym for "My dps is lower than I want" or even just a vague "My dps needs to be overpowered now, because of, you know, scaling."
TLDR: many players worry far more about scaling than they need to. ;)