I can kind of see where you're coming from, but IMO there's a better way to look at it:
a) if you leave your money in CPF OA, you get 2.5% interest p.a.
b) if you use CPF funds to invest via Endowus, you (ideally) want to get >2.5% returns. Now, Endowus offers various options with differing risk-return profile, so you have to determine what is your risk appetite, what is your investment horizon etc., and then see if what Endowus can offer is suitable for you. There could be portfolios that give say ~3.5% avg returns, with a low level or risk, or portfolios that give ~7.5% avg returns, with a higher level of risk.
c) it may also be attractive to consider the one-way transfer from CPF OA to CPF SA, so that you earn 4%, rather than 2.5%. Over the long-term, this makes a big difference, and I guess most of us will view the 4% rate is being guaranteed (i.e. no risk) although in reality, CPF can change it. This options also depends on whether you have plans for your OA monies (e.g. to fund housing purchase, etc.)
For me personally, 2.5% p.a. is low. I am quite young, and my investment horizon is long. I am prepared to stomach risk, because I feel that over the long-term, the average returns will exceed 2.5%. A few years ago I transferred some OA to SA. There was no Endowus option available then. Currently, I prefer Endowus because I feel that in the long-term, the portfolio I am invested in has a good chance of exceeding 4%.