Thaumcraft is a major mod on account of the large number of add-ons as well as its own content. Think of a major gas giant like Jupiter or Saturn with their huge number of satellites. Not only that but some of those satellites act as links to other major mods. Forbidden Magic, Arcane Engineering, Essential Thaumaturgy, Technomancy, Thaumic Energistics, Thaumic Equivalence, ThaumcraftGates, and perhaps a few I couldn't find served in that role. Only Buildcraft and Industrialcraft can boast that kind of clout right now.
That's why so many people are disappointed at Thaumcraft's departure, and why some of the authors of the satellite mods might want to team up to bring the main planet along.
I fully concur about the seismic shift from 1.7 to 1.8 and beyond. Maybe it's time someone created a library that obfuscated the jump in such a way that if a mod author linked to the 1.7 version of the library, then simply updated to the 1.8+ version, it would work. The needed changes are common across a lot of mods and perhaps it's time to create a common way for everyone to switch over.