Yeah, I think someone straight up lied to you about the 20%. That's insane as a ROI from stock over the long term. And stock is usually considered a high risk investment, so NOT the kind of thing that regular citizens use as the main basis for their life savings. If you take 5% as the base rate and a starting investment of $100,000 then you get a cost for resurrection (after 100 years of compound interest) of 1.8 million. That's a far more reasonable figure which fits with what the RPG provides in terms of costing.
No, you don't put your Life Savings into stocks (at least not close to retirement, and definitely never into a single stock or even a single industry). But for basic capital growth, there's nothing wrong with a properly diversified portfolio. Let's take a look at the 2008 Financial Crisis. The Dow lost 50% of it's value. Holy shit, amiright? Not if you're marking time in Limbo just waiting for your capital growth to pay for your Resurrection. The market losing 50% of it's value in a crash only means that you need to wait for one additional doubling of your Resurrection capital. 7% like the S&P500 has made means that's an additional 10 years waiting (Rule of 70). What's an additional 10 years when you've already been waiting a hundred, and aren't conscious for it anyway?
Yeah, but a properly diversified portfolio isn't yielding anything like 20%. 5% is generous, and gives the results I listed above. As this also fits with the costs provided in the RPG that makes perfect sense.