You're right! No good mods are on 1.8.9 at all. We should just forget it even exists.
You know, maybe that's not such a bad idea.
Et Futurum + 1.7.10 has most of the features of 1.8.9 (just missing the attack fishes). So regard 1.8.9 as a testing platform, for dealing with the major under-the-hood changes that Mojang has come out with. Regard 1.9 as the next "end user" version.
Yes, I mean officially adopt an "every other version" strategy.
What does this force/enable Forge to do?
1. Force support for altering blocks from version to version!
Moving from a 1.7.10 world with Et Futurum to a 1.9 world will require mapping Et_Futurum:diorite to minecraft:diorite (and I'm sorry if that's not quite the internal names). Etc. For all the stone types.
Well, if you can do that, you can do other things.
Suddenly, removing a mod, even a worldgen mod, is no longer catestrophic. Imagine being able to change all of the *:copper blocks to universal_minerals:copper, and eliminate all of the oredict headaches in one blow.
2. Got lots of crud accumulated in forge's names over the years when people didn't have a clue, and then didn't want to fix/rename because everything would break? *Break it now!*, in the developers 1.8 release, so that when the next user release (1.9) comes out, it will be working properly.
This is the time to say "Lets fix all the bad API's". All the bad method names. Put in proper argument names for arguments rather than relying on "If we fix these, we break everyone".
Break it now, during the developer phase, so that it will work properly in the next user phase.
3. End user / modpack stability.
How badly do you think end users want "cutting edge", versus how badly do you think end users want to play the game?
If you have the philosophy of "always update to the newest", then players can't get a stable play experience. The whole idea of "No more updates or improvements for 1.7.10, and now we begin months of getting everything to work right together under 1.8.9", with mods getting little or no love for the stable worlds, and modpacks being put into a position of "Hey, be obsolete or be incomplete", well, that's not a happy camp to be in.