I dunno. Negotiations began during development of Shenmue 3, so unless they knew it was going to bomb way ahead of release it seems a little odd that they’d already be planning for it to fail. What’s more, assuming the first season ends in the cave (which the anime seems to suggest that it will), I can’t see them stretching Shenmue 3 into a full cour, meaning that any theoretical season 2 would likely comprise of content from both Shenmue 3 and Shenmue 4.
Releasing the story ahead of time doesn’t seem like the best way to pave the way for a game, so if this were the case I’d be more inclined to believe that the anime was always conceived as a way of wrapping up the story without the need for more games. Granted, we’ve been told that that definitely isn’t the case, but I think that the end point of season 1 will provide us with a more definitive answer as to what their intentions were.
If season 1 ends halfway through S2’s story we can perhaps assume that the plan was always to release two seasons of the anime to cover up to the beginning of Shenmue 4 (season 1 taking us to the halfway point of Shenmue 2 and season 2 to the end of Shenmue 3). If season 1 ends in the cave, I’d guess the plan was either to provide a less archaic entry point or wrap things up entirely. I guess it’s also possible that they’d hoped to have S4 ready for the release of season 2, but this would seem like wishful thinking on their part.