Working orders in a financial exchange can stay around for days, or even months, as part of the working set. For example, waiting for a futures contract to expire. Customer accounts are also similar. By "working set" I mean deals/orders/sales etc. that are currently active. Once a deal is complete it is then part of historical data.
Memory systems are now so large, i.e. hundreds of GBs in a single server, that the working set of almost any business easily fits into memory. Also available memory size is increasing at a rate much faster than any large business is growing.
The scenario you describe is not really an issue. What can become an issue is when you need to hold all historical data for which a traditional database or file-based system is more suitable.
A simple exercise is to calculate the memory required for active entities, or working set, and then compare that to what is available in modern servers. It is possible to keep 100s of millions of active entities around in memory.