If it changes absolutely nothing about the way you play, it is not a mechanic. Since ARR, there has been no skillful maintenance of GL required to perform well in raids. Unlike in T9 and perhaps T7, in HW (with GL's extended duration) breaks have been either so long that TK was just free potency or so short that TK wasn't viable. At that point, it was a mechanic that rendered yet another skill niche, a mere compensation for an above-average ramp-up (a tax more than a "mechanic") over which you had no effective control. Only with later SB did it finally enhance the toolkit rather than merely detract from it, by offering conditional weights, values, and points of decision -- finally becoming a mechanic. Whether that was intended or not has absolutely no effect on the result. It happened, and it was the most interactive and effective design GL had yet taken, especially since its 10-second duration back in ARR.
None of what any of us are advocating for exist. That's why we're advocating for them. The only difference is that what I'm asking for has previous existed, before being effectively removed by changes to GL and DK.
And I'm all for that. I'm just not for not allowing speed variance to ever be a mechanic, as by making it a passive rather than a manipulable. That's not to say it's currently a manipulable. That's not to say it ever has been, except in the hands of particular SkS breakpoints in combination with RoF's slow. But it has been used to good effect at many SkS breakpoints during the times of ToD/Fracture and easily could be again with even minor changes to GL and a rework of DK/LF. Heck, Meditation and SSS could easily be pulled out of the mud by those same, relatively minor, changes. Not that I would stop there.
To me, Monk should have burst, but burst largely of their own making rather than merely on set timers. Set timers can still push the rotation forward and demand sync therearound, adding to rhythm and giving clear initial targets for optimization, but the complexity of Monk should come from decisions made to prepare for an around those targets rather than on just hitting them on cooldown (making what is usually the "lull" the most cognitively engaging part, and the "highs" merely a sweet, sweet drag of satisfaction as it all clicks as intended).
In my preference, it should be wholly viable, but not quite optimal, to stay at full speed at all times, if one wants to play more consistently. No gambling, no huge risk-reward systems that would ever have you drop speed, are required for getting even to the 80th percentile or so, or doing a good 90% of optimal damage.
The rule of thumb for all but the highest levels of gameplay could be "Tap into GL(/Ki/Chakra/whatever), but never so much that you level down (to a lower stack count)," just as "Feel free to burst into a lower GL count if your Twin/DK/Demo durations would otherwise be excessive and you have no further oGCDs to cast within the rotational string" might be a nuance learned early on that adds to, rather than detracts from, flexibility and nuance of play.
On all other parts, I agree with you. I just think manipulating speed could be a hugely lucrative mechanic for Monk if it were just utilized as an actual mechanic rather than a passive with a ramp-up "Monk tax" that enforces a bunch of otherwise bloat skills.