[x]

The more varieties there are, the less likely it is that one disease could wipe them out.

Also, we don't want wild plant species going extinct before we've had a chance to check them for medicinal properties.

[Wuli]

In general you are right.

But (a) in the immediate sense coffee lovers have nothing to worry about and (b) coffee growers and wholesale buyers are aware of the problem also and will over time mitigate its effects, in their own self interest. Adapting to change is something humans have never been as capable of as we are today.