Opinion time!
1. People love(d) IC2 because it was straightforward, had a clear sense of progression, didn't require a large variety of materials to build stuff, and was a core piece of the modpacks it came with.
2. Myself included, I think there's a lot of hesitation to adopt these new mods because the information on how to use them is so much sketchier than older mods like Forestry or IC2 that have proper, useful wikis for them, rather than the patchy mess of stubs that things like Galacticraft have, not to mention many of the mods having a much higher barrier of entry than the old, simple mods did.
2B. Expanding on that point, you'll notice a lot of people are quite eager to adopt stuff like Dimensional Doors and the new pressure plates, because they're simple, straightforwards, and the documentation on how to do stuff with them is easy to find and complete.
Additional bit: There'd also be a lot less pushback against getting rid of IC2 if there was more "Its okay, look at all the cool new stuff you can do with the replacement mods!" and less "OMG STFU YOU FUCKING IDIOT GET THE FUCK OUT WAUG WAUG"