How would you feel about being locked into that 0% rate for 20 to 30 years?
The rate on T-bills only got that low because 1. they're extremely safe, 2. they're only for 4 weeks, and even a 5% rate is only 0.4% over 4 weeks. Selling stocks that are sliding and simply preserving your money's value for a month or two isn't a bad idea if you think the alternative is likely to be worse. Selling 1000 shares for $100,000 and buying 1111 shares for $99,990 a month or two later will make you a nice 11.1% profit when the price goes up again.
OTOH, selling your stocks because of a temporary slump, and guaranteeing that you won't earn more than 3% over the next 20 to 30 years is a really bad idea. Locking up new money isn't any different. In the October '87 crash stocks fell about 20% in one day. They recovered that value in 2 years, which was a 25% increase. If that drop had taken a month and you could have gotten out halfway through with 90% of your value and "invested" it at negative 5% for a month you'd have done even better than those who just rode it out.