Yogscast gets a bad rap due to the number of kids and trolls that stalk their videos, but that sort of thing is expected when you have as many subscribers as them. They have a huge amount of pull in all aspects of the community, not just immature kids, so them switching over to FTB is the biggest thing that could happen from a marketing perspective, even bigger than say, a shout out on the MCF. Not only that, imagine if they had someone who was actually good at this help 'em out? I mean, its a tall task, but if DW20 or Mead or any of the others were featured on the yogscast, their subs would double over night.
From an entertainment perceptive, tekkit is one of the best things yogscast ever did, the ever-looming danger of total destructive war while Simon and Lewis continue care free jaffa production is quite the juxtaposition, and switching over to FTB would mean that rythian (the servers resident mage) would finally get his hands on Thaumcraft.
TL;DR Tekkit will never die unless the yogs let it, they control so much of the community, we need their support
Personally, I don't think that this should be done as I can easily see even someone as patient as direwolf20 (I say this as he has still refrained from cursing out RichardG on camera all this time) would eventually get tired of the yogscasts antics. I would also like to point out that the yogscast heavily used EE to make virtually everything in their series(I haven't watched much of, so if I'm wrong, please correct me) and they would likely experience some sort of shock at the dissapearance of it.