It seems like quite a leap from your premises (1) and (2) to “socialism”. You don’t clarify much what you mean by it in this post, but afaik I agree with your premises but don’t agree with socialism.
One reason is that I am personally interested in being altruistic, but:
I don’t want to force other people (or even myself) to be “altruistic” against their will
I’m not convinced that governments spend their resources very wisely
So “try and increase the size of government” isn’t that attractive as an end goal.
Another reason is that the actual track record of well-intentioned communist revolutions seems extremely bad—neutral at best (Cuba?) and catastrophic at worst.
Did Open Phil shift funds away from longtermist causes when FTX funds became available?