I would love to see that 20M lifted.
However our B spec drivers have the ability to hold 20M each for a total of 100M and can be used as a savings account. Trouble is takes forever to build up these accounts.
The OCD mostly refreshes once every week, most preferably weekdays. But there is no real schedule of when PD actually does refresh it, so It remains unconfirmed.
The 20 million Credit cap and the reason of its existence has many answers, and every one is just an assumption, so there is no real reason. But possibly just to obtain the Penniless trophy (I got it when I bought the 330 P4), or to make the game more..., say "interesting" so you won't buy every car in the game with one stack of cash (not sure how to explain it), or have a "locked" goal to prevent never-ending earning (not sure how to explain this, neither). Maybe more.