I think the things you bring up a reasons why the effect that I mention might not be as strong as one might naively expect, but that's not a reason to expect that it wouldn't have an effect at all.
Personally I think it was a mistake to publish this without speaking with Open Phil. They specifically recommended against what you are suggesting here:
In many cases, we find a funding gap we’d like to fill, and then we recommend filling the entire funding gap with a single grant.
But if an organisation has a policy of holding 3-5 years worth of reserves, this implies that for every dollar donated which is used on its activities in a given year, another 3-5 dollars worth of donations simply ends up sitting in a bank.
An organization could have a lot of reserves but still have revenue and program expenses roughly equal, no?
Could you say more about what you’ve done to validate the 18 month cutoff you are using? Looking at standard practices seems like a reasonable place to start but may not be the end of the conversation. What if most charities have less reserves because of pressure from funders and not because that is the operationally optimal amount?
GiveWell for example funds programs up to three years in the future. Have you spoken with anyone at GiveWell to understand why SoGive and GiveWell have arrived at such different thresholds?
Thinking hypothetically, it feels plausible to me that there are many programs out there that need more than an 18 month runway to fully implement. For example, GiveDirectly fully funded their basic income program from the start, even though the funds would not be fully distributed for 12 years.
Oh hey, I wrote a blog post (sorta) about this.
The TLDR: Since I was a teenager I've been looking for ways to give effectively, and once GiveWell appeared doing so became a whole lot easier.
What Founders Pledge calls “investment-like giving opportunities” are very different from what GiveWell calls “flow-through effects”. The latter is about how (for example) a family that doesn’t have to deal with a child getting malaria may have more resources to deal with (for example) girl’s’ education.
The Founders Pledge analysis seems to omit consideration of flow-through effects (essentially, the idea that charitable impact is also compounding, not only investment returns), which I think would make investing to give look significantly worse.
It’s a very useful signal as a hiring manager, assuming you trust the reference, but also leads to problematic incentives where people feel like they need to spend resources on being liked by those giving references in order to get a job. “Spend resources” here can be understood to include a wide variety of behaviors, including good things like working well with others, innocuous things like networking, but also dangerous things like flattery or (for example) not advertising that the giver of references is a harasser.