Three days ago, during Black Smokers.
Of course it preserves difficulty. If Renzokuken becomes a single button you have halved the number of unique inputs necessary to correctly perform it. It would be akin to taking NIN Mudra's and instead of splitting them into Ten/Chi/Jin and Mudra buttons, you just have "Hyoton" or "Suiton" or "Doton" buttons that execute them for you. Having them as Ten/Chi/Jin both increases the skill necessary to perform them correctly while also cutting down on button bloat (since w/o T/C/J as unique skills to combine, you'd end up with 7 unique buttons to push instead of the 4 we get with T/C/J plus Mudra). But there is a fail-state in T/C/J usage, in that using too many or not enough will result in failure or a different Mudra than the one the NIN wants. Thus, when you pull it off correctly, you feel good because you've overcome a job challenge that now allows access to a powerful skill. Certainly better than if you just pushed a single button on cooldown.
On it's own, the Renzokuken combo isn't much different than any other 1-2-3 combo out there, but taken in the context of other stuff a tank is expected to do, it's fine as it is. Furthermore, even if the combo was consolidated down to a single button, this is another two buttons that may or may not be filled. You certainly can't add more damage to GNB because they're fine where they are now, and while their mitigation and utility may not be the best out of the tanks, it's certainly not lacking. So in other words, this change would do nothing but lower what little skill ceiling currently exists for tanks. Ironic, considering the calls in other threads for more "engagement" and "complexity" to be added for tanks, while this thread is specifically asking for a job to have some complexity shaved off.
Here's where I come from - personally speaking - when talking about many of the "issues" brought up on these forums.
1 - tank balance is the best and tightest it's ever been since ever
2 - any change to any part of a tank will disrupt this balance somewhat
3 - an argument can be made that tanks are not as engaging as they could be (or once were) due in large part to the overarching design of certain tank aspects (emnity being the most obvious) as well as the current state of encounter design
4 - arguing for increased tank engagement/complexity isn't necessarily a bad thing as long as:
5 - it respects the fact that balance between the tanks is rock solid as it stands now
Hence, any suggestion I see gets filtered through these criteria. I find it hard to square the circle of people complaining about a lack of tank complexity also advocating for a tank to be made less complex. Yes, there are accompanying suggestions of how skill consolidation will allow for "other" stuff to be added, but this - in the case of GNB especially - runs the risk of taking that delicate balance we have achieved and throwing it out the window. You can't add more damage to GNB without making it clearly stronger than the other tanks, and you can't add more utility/mitigation to GNB without making it clearly strong than the other tanks. Thus, consolidation of skills does nothing but lower the complexity of playing GNB for no other reason than to make it less complex. There is nothing that could be added in to replace said complexity to keep GNB at the same level, or improve it, because to do so would throw tank balance out of whack.
Of course, you could add stuff to the other tanks, but now we're no longer talking about "consolidating" a single combo on one tank and instead have opened up the entire can of worms that is intra-role balance. This is made doubly complicated by the fact that development resources are finite. We can all agree that GNB is in a pretty great spot right now. DRK and WAR are in far more (relative) need of help than GNB or PLD. Not to say GNB and PLD are perfect, but with the points above in mind, it would be better to focus energy where needed.