They can make any design decisions as they like. But lets be fair and think what these changes will lead to. Take a look at the existing evidence - live meta. Certain classes like guardian druid, holy priest, arcane mage, etc. are branded as BAD by the community and your chance to get an invite to a random pug with them is extremely small. This is mostly due to the community knowing these classes cant do what the top classes can, e.g. rogue, dh, fire mage, prot warrior, resto druid, etc.
The information on the above is mostly from streams, MDI, public posts. Now imagine Shadowlands: each class will essentially be categorized. You are now either a 5cap/8cap/Uncapped DPS. While previously classes were declined because they didnt bring a utility or enough DPS, we now have a picking order: Uncapped > 8cap > 5cap. I dont mind changes, but lets not pretend these changes will not massively affect any of the classes which cant dish it out big time.
And yes, some classes have had cap for a long time. Guess where on the totem-pole they are - yes, at the bottom.