It would appear that the main reason behind HBS releasing yet another stand alone version of the game (despite them saying the contrary) is that they have separated from Maximum Games and will be publishing TGC 2019 themselves.
To further develop TGC2 would likely mean they would still need to be working with Maximum Games, but as this relationship seems to have come to an end, the only move for HBS was yet another stand alone release. I guess what folks will want to see is continued work on TGC 2019 and not see an announcement for TGC 2020 next year.
Splendid analysis on your part Daz... I think you nailed it!