You’re referring to the transmission-virulence trade-off, which suggests if a pathogen kills people more efficiently, it’s less likely to spread. It’s one possibility, but it is not the only one, and we simply do not know whether this coronavirus will evolve to be more deadly or less so, or if its virulence might not change. To suggest any of this points to a laboratory genesis is total speculation, and while armchair scientists like to promote that notion, the actual experts who study infectious diseases have debunked it.