Back in WOTLK or BC, raids lasted a fair bit but there was one and then 2 difficulties.
There wasn't the feeling of running the same raid over and over in the same week.
Now if you have alts, there are so many different difficulties.
For the average player, you end up doing LFR, Normal then heroic on your alts and sometimes also your main at the start of a new raid.
To me it ends up making a raid get old a lot quicker.
With Ulduar there was progression each week and it felt meaningful. With current content, heroic just feels a little tighter than Normal but it's more or less the same bosses. Mythic a fair bit harder but yet again, a very similar boss fight.
I don't see this current style of raiding to hold up as well as it did in the past.