Exactly, vanilla should be a first step. Not a blast off into low earth orbit. I, and others who have commented here, would like to have time to enjoy vanilla before moving on to the expansions. Enjoy the scenery as it were. Not rush through things.
No what I mean by this is that a faster progression means you also have to work faster in each phase to get caught up on the content. With slower progression, there is plenty of time to take it slow, level alts, take breaks from WoW, etc. Further, it somewhat negates the option of playing on x1 speed if the phases move on before you can get caught up.
I've never said to spend 5 years on vanilla content, only to consider 1 year or 1.5 years like classic. If you plan for a slower progression, it can always be sped up if it becomes necessary. Progression is great and is the main draw of this server, rushing through content is not.