I'm just going to come out and say it who thinks DK mobility is 'fine' and/or "always imagined dks as a slow unstoppable juggernaut" either doesn't play high-end PvE or started playing DK after legion, it's that simple.
The fact is DK's are underrepresented at mythic level because their mobility is shit and they have very poor CC and utility (and on the whole middling damage and even when our damage is 'good' it's normally aoe/cleave damage not st). This is especially true for high mobility fights, it is simply the case that DK moblility is not 'good enough' in circumstances like that.
The CC thing was the whole debacle with mass grip being too centralizing for encounter design (which is fair as far as dps goes) which we've never really been compensated for in any way, it's possible (and even likely) that the reintroduction AMZ will go aways to ameliorate this.
As for mobility. Mobility is one of those 'invisible' stats people don't tend to think about at all until they don't have enough of it and on the other hand, once you have 'enough' it falls off the cliff of diminishing returns. As you can see in a fight like wrathion Dk's don't have 'enough'.
MuH hUMuGiNeYeSaYsHuN
Compare a rogue kit to a DK kit to see how sorry it really is.