Fuck me a wrote a long answer and it didn't get saved in the respond mode. That sucks;

Ok so here is the short version:

From the outset, the idea of a mosaic backbone is pretty nonsensical.

Several reasons:

First; what is the research question such a construct could address that would warrant it's creation? There are already some SARSr backbones to study/optimize human ACE2 binding affinity or introducing cleavage sites. Your proposed backbone (stitching together different pieces from different viruses) just risks creating a viral backbone that is non-viable; this has to do both with potential disruption of functional RNA elements as well as viral protein-protein interactions. Studying viruses requires them to be in their context. Researchers also try to investigate a specific thing, altering backbones would make any study of ACE2/FCS impossible to detangle from backbone effects; so they'd have to spend years first establishing the biology and features of their synthetic backbone. (and this would have been published in between, but that's another topic)

This is btw exactly what Baric's team did; and their scientific reasons for creating this backbone were quite clear: They couldn't get the SARS-1 like bat CoV sequences to work in culture, because of sequencing errors, so they used the bat 'consensus' to basically recover what sequencing might have messed up. Did you read their paper since you even quoted them for this article?

Second, by what metrics were the bits and pieces chosen that 'made' it into the desired synthetic backbone? And why chose 3 viruses arbitrarily to create said consensus backbone? Why not 4? Or 10? Or the whole of the sequences SARSr family? SARS-CoV-2 was not even known at the time these experiments would have needed to happen; so chosing now 3 candidates that closely resemble SARS-CoV-2 as the 'starting' point is not only questionable, it's bordering impossible given when these viruses were actually sequenced.

Ok before I lose it again, I'll continue further down with some more points.