To clarify my point.
When scientists speculate about what LUCA was like, they are
not doing so by saying: "Abiogenesis is true, and Darwinian processes reigned between abiogenesis and LUCA, therefore we can deduce such-and-such a thing about LUCA".
Rather, they are saying: "The observations we make of modern organisms are true, and Darwinian processes reigned between LUCA and modern organisms, therefore we can deduce such-and-such a thing about LUCA".
And
you agree with their premises.
So if you're going to say that FLE predicts
a priori a non-minimal genome for LUCA, then what you would
want them to say is that their interpretation is that LUCA had a non-minimal genome.
But I think you're wrong about what FLE predicts. I have no difficulty conceiving of aliens (for example) seeding our planet with organisms with a minimal genome, which then evolved in a Darwinian way to produce the diversity of life we see today. I have no problem with this because I also have no problem with believing that LUCA had a minimal genome, was
not seeded by aliens, and produced the same results by the same mechanisms.
The question of whether LUCA was or was not minimal can be decided, if at all (which I doubt very much) not
a priori from the competing hypotheses of Darwinism and FLE, but
a posteriori from the data about modern organisms that we now have in front of us. Which totally screws your argument.