I'm not sure I understand your point there?
We have not dropped support for 1.7.10 or any later version. We are not adding new features to GC3 (1.7.2 and 1.7.10 versions) but we will still fix any major bugs identified in those versions. We are slowly, adding new features to GC4 (1.8.9, 1.10.2, 1.11.2 and 1.12.2 versions all have the same features). But, I have recently spent a month of development time which could have been spent adding new fun features for GC4, instead updating to 1.12.2.
The only significant reason to update to 1.12.2 is if you want to include Galacticraft in a modpack with another mod which is only available in 1.12.2 (or which has its best version in 1.12.2). Apart from that, there is no difference between the 1.12.2 version and the 1.11.2 version of Galacticraft.
Some mods are not yet updated to 1.12.2 at all - for example, EnderIO, Minefactory Reloaded.
Technic platform's most popular modpacks are mainly for 1.6.4 and 1.7.10. There is nothing wrong with those versions of the game. 1.7.10 especially is nice and stable and most mods for that version are fully mature - meaning fewer bugs.