I’m sure they’ll eventually get taken down, but the issue is that many of the show’s most die-hard fans will have already seen them and watched them on YouTube the day they came out - just look at Eurobricks, where there was a discussion on each episode immediately after release. Will some people who participated in the discussion see it again when it airs? Probably. Will everyone? Definitely not - there’s at least a substantial number of viewers who will have been satisfied watching the first time around. As to how big of an issue it actually is, I’m not sure - this happens every season to little effect, but the fact that TA actually retweeted it and drew it to everyone’s attention means at least he is concerned about it, which is a little worrying.
I'm not really sure why he would be concerned about it, by all accounts it really shouldn't be that big of an issue. If Ninjago's doing well enough to have continued this long, the amount of fans who don't watch it because they'd already seen it aren't going to be enough to cause it to be cancelled. And the franchise is the sets first and foremost anyway (or at least, I'm pretty sure that's how this works) so everything should be fine. As long as the show isn't terrible enough that it wouldn't be worth continuing if the sets can apparently sell on their own, but at the point, fans watching the early releases of the episodes wouldn't be the cause of that, nor could they have been enough to save it.
If the show does do less than satisfactory on cable, there are plenty of factors that cause that, but it isn't because of the recordings on YouTube.