GiveWell is on the case, and has said it is looking at bio threats (as well as nukes, solar storms, interruptions of agriculture). See their blog post on global catastrophic risks potential focus areas.

The open letter is an indication that GiveWell should take AI risk more seriously, while the Musk donation is an indication that near-term room for more funding will be lower. That could go either way.

On the room for more funding question, it's worth noting that GiveWell and Good Ventures are now moving tens of millions of dollars per year, and have been talking about moving quite a bit more than Musk's donation to the areas the Open Philanthropy Project winds up prioritizing.

However, even if the amount of money does not exhaust the field, there may be limits on how fast it can be digested, and the efficient growth path, that would favor gradually increasing activity.