TLDR: Despite recent progress, is it possible that fusion will reach efficiency where it is net positive in energy output but still too expensive to be useful?
Long Version: When I first heard about fusion, the idea was that this immense energy could be harvested taking advantage the the conversion of matter into energy. Everything I heard was that the quantities are so great that if we could just nail the sustained fusion reaction we could harvest potentially limitless energy.
As I've come to understand it however, it is not so simple. The big question is in how long you can sustain the reaction and how much energy it costs to start it in the first place. It seems to me the strides that have been made over the last few decades are to bring the cost down enough and extend the reaction long enough that the net energy loss is lower, then break even and now possibly a net gain. I've seen some articles imply that this turn of events means fusion is definitely on the table as a near future abundant energy source.
My question is this. Is it a forgone conclusion that the current trends will continue? Because if not, doesn't that mean fusion could still get stuck somewhere where there is a net energy gain but it's still too expensive to be useful?