Lex blocking coremods from modifying FML or Forge classes means that if another situation arises like with 1.6.4 or 1.7.x, where there's a serious bug that will never get officially fixed due to the lack of support for old versions, then not even a coremod can fix it and we'll be relying on JAR patches. You know, the very thing that got me griped at when I released a patch like that for the problem initially, and what spurred Lex to make the coremod version because he said he was tired of hearing people talk about my version. Based on today's freak-out, it was more likely that he just didn't like me touching FML in general rather than my approach to doing so.
Lex eliminating all coremods, on the other hand, means a lot of the big 1.7.10 mods will never get updated to 1.8. Many mod authors don't seem to bother trying to get additions made to Forge anymore, probably because of what they face when they try to, so ASM is the easier option these days. Combine that with the irrational choice Lex made for the rendering in 1.8, ASM is almost essential for some mods. My own would stop working, and I would likely just leave them that way, since I already have one of them fully ported and running on my own API and loader.
The dumbest part about it is that custom renderers and coremods are just not that big of a problem. Of course bugs happen, but bugs get fixed. If a mod is too buggy, we simply don't use it until it's better. For an API to restrict modding capabilities rather than increase them, especially when based on what-ifs, is completely counterproductive and nonsensical.
Even in the latter situation, with all coremod functionality removed, 1.8 Forge's use in the short-term is still viable. Someone could replace the 'main' class in the user's version JSON with a pre-loader, modify Forge before it ever loads to remove the restrictions and/or add coremod framework back in, and be on their merry way. Though I emphasize short-term, because this is a ridiculous situation to be in, and when an API is actively fighting you then it's time to stop using it, or fork it. Most modders seem keen to stay on 1.7.10 anyway, mostly due to the rendering, and that's where Immibis' Forge fork is as well, so 1.8 is looking less and less viable. It's a real shame.