It's easy to assume it's artificial difficulty, but the fact is we don't know how all of these things are coded. Maybe the combo has to be on three separate buttons because if it isn't then interspersing literally anything else will break it just like it does with Continuation. You use ANY GCD skill during during your Renzokuken without first using Continuation and it cancels the Continuation combo for that step. This creates an opportunity for skill to be displayed because it locks you into certain patterns with this combo. The goal when doing damage as a GNB (or anything really) is to keep your damage cooldowns on cooldown as much as possible. There are plenty of times where even a single GCD of difference in Bloodfest means you're sitting there waiting, unable to target a boss, essentially wasting precious seconds that a cooldown could be ticking. It's not uncommmon for a Burst Strike to be thrown out in the middle of Renzokuken (which doesn't break THAT combo, but WILL break Continuation if Cont. isn't used first). This provides a brief moment of respite from the normal pattern of 1-2-3 combo's by now adding in a layer of priority-based complexity, and can be used to "cook" things in such a way that you can use Bloodfest earlier than you would have.
But let's be honest, this thread is simply the "Storm's Eye is a chore" thread with a GNB-flavored tint on it. Idiots won't be happy until they can push a single button and the boss instantly blows up. Then they'll come to the forums to complain how bored they are and how it's time to move on, ded gaem etc. Point being, the difficulty isn't simply a matter of "interface." It's created, on purpose, by the devs, so that we have a hurdle to overcome. If you want one-button wonders, try Cow Clicker, or any of the other mindless clicker-games out there. Yes, difficulty in this game is contrived. All of it. That's why it's a game. Goals have to be set for us, challenges created that we overcome, and difficult trials created that we can weather in order for us to have any feeling of accomplishment.
The inverse of this request is to ask SE to just have every boss stand still and occasionally use a raid-wide aoe (to proc certain counter-skills like Riddle of Earth you see) while we just wail on it for 10 minutes. It's "bad interface design" to have to use non-combat keys like WASD to waste time moving around when I could just be spamming my one god-button that plays the entire game for me.
Which would be stupid of course. Just like this request.