You are correct. The single lineage argument is unconvincing because it is yet another example of false inference.
The LUCA study showed that currently known terrestrial life forms have a common origin in a hydrothermal vent prokaryote. But, prokaryotic cells already manifest very substantial structural complexity in the form of double plasma membranes, cell walls, pilli, flagella, cytoskeletons, nucleoids (self-replicating DNA), ribosomes (self-replicating RNA), plasmids (self-replicating rings of DNA). These cells communicate environmental conditions to each other via chemical mediators (quorum sensing), and exchange genetic information with adjacent cells through the pilli (e.g. antibiotic resistance).
Numerous structural elements, dual genetic composition, regulated gene expression, and behavioral sophistication indicate that LUCA itself was not a primordial life form, but rather a merger product of MULTIPLE precursors.
The single lineage argument is easily refuted by:
- Incomplete investigation of all terrestrial life forms,
- Extinction of selected lineages caused by interspecies competition or environmental factors, and
- Merger of multiple primordial lineages into more complex biological
I think this is really overstating the case.
None of your bullets are definitive evidence of multiple starts, just evidence it's possible and plausible.
In reality, this is just something no one is sure of.
On the face of it, there is one form of life today, so it is reasonable to think there was one start. This is also supported by the manifest unlikeliness of it happening.
However, the speculation that it could have started more than once, and one form won out, is legitimate. Certainly there is no evidence this is not the case.
In between, there is the idea that current life is an amalgam of multiple starts. This has a good deal of circumstantial support. The issue for this debate is what do you mean by the start of life?
At one end, you have some separate chemical paths that are clearly not life, fusing into a single life form. That sounds implausible to me.
At the other, you have multiple distinct life forms fusing into a symbiote which wipes out its antecedents. That also sounds implausible to me.
In between is a vast, much more plausible, spectrum of possibilities where it is not necessarily obvious exactly where the moment life starts. In this area, you are arguing about semantics as much as science.
And, as I said above, we simply don't know at the moment.