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At the moment di2 105 would sit as close to Ultegra as to be too similar price-wise
That's not an impediment in itself. When I was a boy, there didn't seem to be an exploitable gap between the BSO grade Tourney parts and Golden Arrow 105, but Shimano have dropped Tiagra, Sora and Claris into that space.
The decision isn't based on any absolute or relative price difference, it's based on price elasticity of demand. If nearly everybody who wants electric gears is also insensitive to a couple of hundred pounds difference in initial purchase cost, then there's no need for 105Di2
Shimano is not interested in a price war with itself. 105 Di2 will only happen when makes more money for Shimano than it loses them by cannibalising marginal Ultegra sales.