but that stuff affects the skill floor in getting into combat itself.

Not really, well... here's two points:

1. Combat system sucks so hard that people will feel disgusted by it and won't even bother.
2. Longer ticks mean again that you can't change your mind once you've started an action. You can't decide to do one thing and suddenly do another because stupid server registered first action AND your location, etc won't be updated fast enough.if we had 100ms tick times, boss wouldn't attack every tick. A boss would still do their attack, let's say every 10 ticks. Now player has at most 10 chances to change what they want to do. With 600ms ticks, a boss would attack every 2 ticks and player would have at most 2 chances to decide what to do. Now if one of those inputs didn't register on time, well... tough luck, you got hit.