To claim Factorio Space Age as counter example would require to show that it was as successful as DLC as it would have been as a new game.
Probably not easy to show...
I think that's impossible to prove and I don't see how it's relevant. The OP claimed that people aren't willing to pay full price unless it's a clear, separate sequel with hype around it. The devs of Factorio proved otherwise with 400k copies being sold in the first week of Space Age's release[0].
0: https://factorio.com/blog/post/fff-434