I can only speak for myself, but I don't think I would have the motivation to start modding on a platform knowing that the next version of the game will be out before my mod.I wouldn't worry about that. If they release the source code, people will be messing around with it for years to come. I expect considerable reluctance from players to move from CIV VI to a new title, perhaps even greater than what there was between V and VI.
I've stopped modding civ5 not long after Firaxis stopped to work on it, knowing the platform was imperfect (no mods in MP for example) and waited for civ6.
Now if they tell us of their development plan and that plan involve perfecting the civ6 engine (which, if the DLL source was released today, is still in some ways worst than ci5 and civ4 for my use ATM), with civ7 being more than 4-5 years away, yes, it's not too late to release the source code.
For Humankind, it's wait&see, they've not said anything about modding yet AFAIK, and their past games seem to be less open to modding than civ4-5-6.