Agreed, some of the reasoning for extraterrestrial origins of viruses could also be explained by terrestrial processes. For example, the fact that new viruses are being added as quickly as old ones degrade and disappear doesn't necessarily mean alien viruses, but could also be reservoirs of ancient viruses being released into the environment or rapid mutation of existing viruses. Viruses having an extraterrestrial origin implies either RNA is spontaneously self-generating in the universe in large quantities, or somehow getting carried over across interstellar space from an alien ecosystem, and there's not a lot of evidence for either case. But given that viruses can survive intact for long periods of time since they don't have to perform biological processes (and maybe even longer underground away from degrading UV light) and the vast diversity of life that has lived and gone extinct on this planet, I'd be more inclined to think we're getting infected by Mammoth diseases rather than Alien diseases.

This theory is less convincing when you consider that viruses don't seem to share any common genetic ancestry with organisms living today, but they could share genetic ancestry with extinct organisms that didn't leave leave any living descendants with genetic mutation adding even more obfuscation to their origins.