With the invest option you can get your money back at any time as well. So say I invest 100SBD I might want it back in say 5 days, I put reinvest at 100% and I get 100.5 back instead (I don't know how much I would actually get back but it would be more). This just wouldn't work with anything else on steem I have seen.
With the proposed change this is not guaranteed but the system mentioned is exactly how your bank works, i.e. they don't have the means to pay everyone their balance in cash but not everyone want cash at the same time so its okay. Of course a high percentage of cash to investment ratio is needed to avoid issues but I would imagine 1 STEEM liquid cash for every 2 STEEM invested is very feasible and much better than any bank.
In my opinion 50% would actually be pretty conservative, but then again I haven't analyzed what their account turnover, etc. is. If people hold for long periods of time then they could probably get that closer to 20%. Also based on how active they are, if enough people are investing in as people cash out while getting paid sbd for votes it could run very lean.
Definitely better than a bank.