If there hadn’t been a boost either
A) fewer people would have played full stop.
or
B) more people would have levelled characters up in the Prepatch (or previous weeks/months because its not as if TBC was unexpected.)
Either way, the boost however has minimal impact on why so few people are playing level 35 characters today. The reason there are so few is because most players have a level 70 and don’t want to do it again. It equally has minimal impact on why so many players are quitting - which is because they’ve hit 70, maybe cleared the raids a few (or half a dozen) times, and now have nothing to do until the next phase comes out. After a few weeks its unclear why you are bothering. You don’t need full tier 4 BIS, there’s a clear argument to just saving yourself a few months of subscription fees until SSC/TK come out.
I think people who didn’t play in the pre-patch missed out on a great experience - it felt like an MMO to have a cohort proceeding through the content at roughly the same pace. There were always people to group up with. But equally, it took about 5-6 hours a day for 2 weeks to be ready for the Dark Portal, and I can understand why many would not be able to make that commitment. I’d imagine being level 45 or something when all your friends (or just 99% of the server) headed through the Dark Portal would have sucked. There’s going to be no one to play with as you slowly finish that grind - indeed there’s no one to meet you even at 60, because everyone’s pushed up to 70.
Starting out today - either fresh or as an alt - is going to be such a worse experience than I had from the middle of May.
Its a similar story with regular WoW Classic. I think that game remains great. But its not great to very slowly grind from 1-60, knowing that unless you start and play at the same pace as a bunch of other people, you’ll likely be alone for 99% of it.