• I happen to disagree that possible interventions that greatly improve the expectation of the long-term future will soon all be taken. But regardless, the first quote is just about value, not about what we ought to do.
• I think the second principle is basically true. Since the long-term future is 10^(big number) times bigger than the short-term future, our effects on the short-term future mostly matter insofar as they affect the long-term future, unless we have reason to believe that long-term effects somehow cancel out exactly. You're right that humans are not psychologically capable of always following it directly, but we can pursue proxies and instrumental goals that we think improve the long-term future. (But also, this principle is about describing our actions, not telling us what to do, so what's relevant isn't our capability to estimate long-term effects, but rather what we would think about our actions if we were omniscient.)

I'm unsure if I agree or not. I think this could benefit from a bit of clarification on the "why this needs to be retired" parts.

For the first slogan, it seems like you're saying that this is not a complete argument for longtermism - just because the future is big doesn't mean its tractable, or neglected, or valuable at the margin. I agree that it's not a complete argument, and if I saw someone framing it that way I would object. But I don't think that means we need to retire the phrase unless we see it being constantly used as a strawman or something? It's not complete, but it's a quick way to summarize a big part of the argument.

For the second one, it sounds like you're saying this is misleading - it doesn't accurately represent the work being done, which is mostly on lock-in events, not affecting the long-term future. This is true, but it takes only one extra sentence to say "but this is hard so in practice we focus on lock-in". It's a quick way to summarize the philosophical motivations, but does seem pretty detached from practice.

I think my takeaway from thinking thru this comment is this:

• Longtermism is a complicated argument with a lot of separate pieces
• We have slogans that summarize some of those pieces and leave out others
• Those slogans are useful in a high-context environment, but can be misleading for those that don't already know all the context they implicitly rely on

I think the existence of investing for the future as a meta option to improve the far future essentially invalidates both of your points. Investing money in a long-term fund won’t hit diminishing returns anytime soon. I think of it as the “Give Directly of longtermism”.

You might be interested in checking out a GPI paper which argues the same thing as your second point: The Scope of Longtermism 

Here's the full conclusion:

This paper assessed the fate of ex ante swamping ASL: the claim that the ex ante best thing
we can do is often a swamping longtermist option that is near-best for the long-term future. I gave a two-part argument that swamping ASL holds in the special case of present-day cause-neutral philanthropy: the argument from strong swamping that a strong swamping option would witness the truth of ASL, and the argument from single-track dominance for the existence of a strong swamping option. 

However, I also argued for the rarity thesis that swamping longtermist options are rare. I gave two arguments for the rarity thesis: the argument from rapid diminution that probabilities of large far-future benefits often diminish faster than those benefits increase; and the argument from washing out that probabilities of far-future benefits are often significantly cancelled by probabilities of far-future harms.

I argued that the rarity thesis does not challenge the case for swamping ASL in present day, cause-neutral philanthropy, but showed how the rarity thesis generates two challenges to the scope of swamping ASL beyond this case. First, there is the area challenge that swamping ASL often fails when we restrict our attention to specific cause areas. Second, there is the challenge from option unawareness that swamping ASL often fails when we modify decision problems to incorporate agents’ unawareness of relevant options. 

In some ways, this may be familiar and comforting news. For example, Greaves (2016) considers the cluelessness problem that we are often significantly clueless about the ex ante values of our actions because we are clueless about their long-term effects. Greaves suggests that although cluelessness may be correct as a description of some complex decisionmaking problems, we should not exaggerate the extent of mundane cluelessness in everyday decisionmaking. A natural way of explaining this result would be to argue for a strengthened form of the rarity thesis on which in most mundane decisionmaking, the expected long-term effects of our actions are swamped by their expected short-term effects. So in a sense, the rarity thesis is an expected and comforting result. 

In addition, this discussion leaves room for swamping ASL to be true and important in the case of present-day, cause-neutral philanthropy as well as in a limited number of other contexts. It also does not directly pronounce on the fate of ex-post versions of ASL, or on the fate of non-swamping, convergent ASL. However, it does suggest that swamping versions of ASL may have a more limited scope than otherwise supposed. 

"What matters most about our actions is their very long term effects."

I think my takeaway from this slogan is: given limited evaluation capacity + some actions under consideration, a substantial proportion of this capacity should be debited to thinking about long term effects.

It could be false: maybe it's easy to conclude that nothing important can be known about the long term effects. However, I don't think this has been demonstrated yet.