I don't see population reduction being good for their bottom line overall.

While one could argue that it takes people out of the wealth redistribution pool, that doesn't necessarily mean that the wealth would redistribute the way they want it to. There could be argument that eventually the wealth would redistribute up to them and only them, but there's far too many variables to make this gamble viable - not with the massive time window that it would take to actually work. Especially when you factor in things that were beyond their direct control; the risk just begins to outstrip the reward far to quickly.

They also didn't - and still don't - have much of a guarantee that they're going to be able to buy their way out of the situation. Space programmes at that point were still incredible moon shots, as were most medicine and technology ventures. Without there being a for sure way to get themselves out of the hellish creation that they'd be making, I don't particularly see the elite trying anything so dangerous as a mass population reduction that wasn't directly tied to some sort of controlled method (and climate change is definitely not a controlled method).

Now, if you were to make a case for the elite using the industrial war machine as a method of population reduction, I might be more convinced, because that is far better controlled than climate change and pollution.