I wouldn’t say necessarily “kill the fanbase”. It would split it, that’s for sure, but that’s not synonymous with “killing a fanbase”.
If it was for me, I’d like to say NO connections between G4 and G5. I’m glad that the Mane 6 are there as little toys that represented a beautiful idea that Sunny and her father truly believed, nothing more. I even defend some creative decisions the G5 made to give the world more consistency, like how the magical stones allowed the ponies to have magic.
If G4 has proven something as much as possible is that unicorns and pegasus were dangerous. Creating a scenario where earth ponies show inventions to prevent possible problems related to unicorns and pegasus (even though much of it is based on gossip) was much more believable than them having magic and the ability to fly right away.
I don’t like operpowered characters. This prevents this issue and I’m glad they did it.