How else are we supposed to look at it, since it very literally is reductive?
reductive
What part of that doesn't really fit the scenario? Did they not remove lots of options from combat, compared to their previous offerings? Yes, they did. Isn't this going to make dealing with combat simpler? I mean, you'll have less buttons to keep track of, so I'm thinking it will. So how dare we look at it exactly as it's presented, we should be wearing our rose colored glasses too.
The combat is only reductive because your argument and stance on the subject is reductive. You're taking a complex system that is developed and informed by many different aspects of the game and acting as if the limit on hotkey abilities is the only thing that matters and thus it's limitation ruins the game. That's the definition of reductive, presenting a situation in it's most basic and overly simple parts until the argument loses almost all meaning. You're missing the forest for the trees.
I've already given a clear example of how the reduction of combos from DAO to DA2 actually resulted in more choice and variety in combat, whether you enjoyed that change is a completely different story altogether as it is a subjective one, not objective. Another would be ME1 to ME2 where they originally tested this out by retooling the whole powers system in ME2, reducing the overall number of powers and skill the player could invest in, but added depth by giving each class a more clear and unique role in combat, as well as making those abilities they did have more versatile and useful like creating warp bombs. Further expanding on that in ME3 when they introduced power combos which added a whole suite of combos across the tech and biotic branches.
Another avenue that hasn't been touched on as far I've read is the fact that the crafting system now in DAI is by all accounts quite deep, especially compared to past games. We are going to have a great deal of control of over not only the visual aspects of our weapons and armors, but all their bonuses and powerups they have by way of the materials we use when creating the various parts of the weapon, like the blade, hilt and pommel. This is in addition to the already well established system of enchantments that we can place on weapons and armor. This will be yet another aspect that will inform the rest of the combat and something players will have to take into consideration when outfitting their party and preparing for battle. Having the good bonuses on your equipment could mean boosting the effectiveness of your abilities so that you take that enemy out in 3 hits instead of 5, you tank will be able to aggro and hold out against 5 enemies instead 4. These things matter and affect combat, your strategies and how plan out your tactics in combat based on the skills and abilities of your party. The game isn't just some puzzle where you match X ability to Y enemy and that's all there is.
Your entire argument is based solely on the number of available abilities and how many can be in the hotbar at one time, which is a very important aspect of combat, but far from the only one.
I think even with 8 abilities they can just fix the whole mess by allowing us to change the 8 during combat as well. Kind of keeps both sides of the argument happy.
If this really is a design choice, which is very much appears to be, then that is in no way a fix, but would likely throw off the entire combat system. It's highly unlikely they simply just capped the number of abilities you can use and that's it with not other changes to how the combat operates and is designed. More likely the 8 ability limit informs all aspects of combat, like ability powers and progression, class progression, enemy and encounter design, weapon bonuses and so on.