The 100-400mm is still a GM lens - not only superior optics, but superior focusing system as well, and likely better sealing. And better build with less/no plastics, compared to the 200-600mm - which is why they can still charge a premium.
For me, the 200-600mm could still be a better choice than the Tamron / Sigma 150-600mm pair for a few reasons. First, I wouldn't say that it can't be better optically - in fact, the Sony has plenty of room to improve on those two for image quality, as both are a bit weak at the 600mm end especially with distant targets. So better optical performance especially at the long end would make the Sony worth considering. Second, the focus speed and performance of a native lens - adapted lenses work OK, and fine for walkaround wildlife, but as soon as it gets into action, flying birds, etc, the adapted lenses range from mediocre and below native performance, to poor...depending on the adapter and lens. No adapted lens approaches the speed, performance, and accuracy with moving targets of a native lens. And third, that internal zoom design means that the lens has better overall balance, and maintains that balance better without protruding - and it won't have the dust-pump action that the extended zooms do. I would definitely consider replacing my adapted Tamron 150-600mm lens with this new Sony 200-600mm lens...and it's a bonus feature that the 200-600mm is compatible with the 1.4x and 2x TCs...so I can stick the 1.4x TC on it for 1,260mm equivalent reach, handheld and stabilized. The Tamron is just not very compatible with TCs, with the optical quality falling far off - the matched TCs from Sony are very good optically, and autofocus should even still be maintained at least with the 1.4x TC.
I'm currently holding off for reviews and tests, before buying the 200-600mm...but moreso I'm holding off in hopes of news of a new APS-C high-performance body - which is really more what I'm looking for to replace my excellent but well used and old A6300...If Sony release a 'mini-A9' with an APS-C sensor and the large battery, body controls, EVF, and focus system tech from the A9, I'd buy both that AND the 200-600mm lens together, and still keep the 100-400mm for maximum optical quality when needed, as well as the vastly superior close focusing ability.