This is just the Politician's Syllogism[0], though:
We must do something.
This is something.
Therefore we must do this.
Are the detractors of string theory saying we should stop doing fundamental physics entirely? I don't think so
My understanding of string theory is that there isn't even a proper string theory variant of the Standard Model, and that most of the models people are trying out are basically toy models that kinda resemble, say, QED. This means it's not even coherent enough to ask the question if it fits existing data, which is part of the premise of it being "not even wrong."
Nobody knows this since there is no exact mathematical formulation of "the theory" to begin with.
> that fits existing data
Nobody knows this either since nobody has derived either General Relativity or the Standard Model of particle physics as an approximation to any string theory model.