I think it's going to be less of an issue this time around because you're not perpetually stuck in progression mode.
Unless you were a high end guild in TBC, you were always progressing through content until you killed Illidan or even KJ, there was no time to catch on with alts, hence you had to give up progression time in order to get trials or someone who just has to fill a spot attuned.
Now, you have months between each tier, people will get their T5 en masse ready during T4, people will get their T6 ready during T5, you have more than enough time to get attunements done on alts (which will be far more prominent now than back then), socials, whatever, you'll have pugs that might be able to clear a given content, the overall rate of people being attuned will be much higher, which in return in makes the need for poaching less likely.
Disregarding that people will clear the content at a much higher pace will also reduce the issues of having to reclear a given raid.
I think it's going to be similiar how people set up raids aside for Ony / ZG for world buffs or Idols, people will be attuning alts & socials on the side for an emergency.