Only a minority of users will ever take part, meaning centralized exchanges which are either already insolvent or running fractional reserve (i.e. all of them) can easily cover the small number of coins which will be withdrawn. Further, since it is advertised and the date well known, all these exchanges can easily pull back in some risky investments or loans, or take out a loan themselves, to cover the withdrawals for a day or two if they need, before going right back to their usual scammy ways the very next day. Despite this day existing for several years, the number of exchanges going bankrupt or insolvent or turning out to be scams and their users losing everything is going up, not down. This day proves nothing and prevents nothing.
Most users who take part will withdraw their coins, pay the ridiculous withdrawal fees, and then just send their coins straight back the very next day. All they achieve is lining the pockets of the centralized exchanges even more, while gaining zero additional security.
If your coins are on a centralized exchange, then you do not own them and you are at constant risk of losing everything. This pointless day changes absolutely nothing. The only real proof of keys is having your coins in your own wallet 24/7/365.