They will be looking at costs and returns though. Expansions are not for free but have to be developed as well. If you trained a horse to make money at horse racing but the horse does not succeed you will not put your money into the horse for its whole life cycle but you will get rid of it and look for better investments no matter how much it has already cost you. If player numbers are an indicator of how many people are willing to spend more money on this game then too few expansions might be sold to justify the costs of developing them for the next years. I agree though that we will see more updates and at least one true expansion before there is the slightest chance for the development of Civ7 being cancelled. If they get rid of eras and civ switching I will even give it another shot but I doubt that is going to happen.
The same would apply to a hypothetical Civ 8, though. It would require even more investment with no guarantee of return either. Especially if Civ 7 gets cancelled, consumers might be even less forgiving for the unfinished hot mess that we all know Civ 8 is going to be on release (if it ever gets made by Firaxis). Trust that the game will be supported and expanded is a big selling point for this type of game.