TLDR; If you pulled 60-90k on your arcane mage for most fights full raid buffed you might want to stick with it. Your numbers might not be that much different.
I disagree. I think mages who weren't pulling great numbers before will be hurt the most by these changes. Look at the delta in 5.1 between median arcane parses and top 100 in 10N:
Top 100: 119k
That's a difference of 46k dps between average players and top players. Expressed in relative terms, a top arcane player outputs on average 63% more damage than an average arcane player. Compare that to something like a frost dk, where the difference is much lower at 43%, and you'll realize that there exists a very problematic gap between "average" and "optimal".
What this means is that Blizzard does not have a good "target dps" to shoot for with the current arcane design. They either aim too high and risk overpowering top-end players, or they aim too low to keep the top players balanced and end up making it a very weak spec for average players (which is what is happening now). I believe the culprit is Rune of Power; aside from all the poor quality of life issues, it's simply too hard for an average player to get the full mileage out of it.
It will be even worse next patch. The haste/invocation style is a confused mess on the PTR and produces much lower damage than the mastery/rune style. Average arcane players will be stuck with a talent that they already cannot fully utilize; in fact, I think the only reason it seems okay now is because the damage is overpowered, which is masking all the true issues with the spec's design. Just look at mostly standstill fight like Feng, in which Rune of Power isn't heavily punished:
Arcane is right in the middle of the pack. This trend can be seen throughout all of t14; only the top end players really push it to being an outlier of a spec.
Now take those numbers above and apply an across the board 15% reduction (a number I've seen thrown around for optimally played arcane, not sure how accurate it is but it seems reasonable). For the top 100: arcane is now middle of the pack. For the average player: arcane is now the worst spec with any sizable representation on raidbots. You'll be looking a drop to about 62k dps for all fights and 55k on Feng, which is below even arms warriors.
My suggestion to any mage at any level is to ditch arcane ASAP. Even for a top end player, the mobility issues are simply not worth it for standstill dps comparable to frost. I would suggest playing frost and gathering crit gear until ~35% crit self buffed, then switching to fire. Frost's dps doesn't suffer much from gearing crit instead of haste up to the soft cap of 28%, so it's easily possible to gear straight fire until you're ready to make the switch.