You can naively extrapolate that, but it will be, well, a naive extrapolation. Not necessarily a bad thing, but it won't necessarily be accurate. If the virus does get into, say, the US, but it happens to only infect 20-40 year-olds through office transmission, it probably won't even be that fatal. If it happens to get into a senior home, it could be a great deal more deadly.

And that's before we consider the spoiler of mutations, which is the real problem. In the long term, it is evolutionarily advantageous for the virus to become less lethal and eventually fade into the background as just another cold, if it isn't wiped out by aggressive quarantining. However, in the short term, many of the same things that will make it more transmissible, such as more effectively converting host systems into virus factories, or contrariwise, being more effectively hidden while still being contagious, will also make it more dangerous to the host and/or society.

But for all that, there is a sense in which the "naive extrapolation" is also the best thing we have right now based on available data. It is, at least, data-driven.