Even if economy remains good, with inflation maintains 3% per year, how can an average person be able to reach the mathematical calculated FRS of 310k in 20 years?

Yes, as long as you don't leverage too much on buying an expensive home and buy a private hospitalisation shield plan with rider (due to hitting the max of your MA account and not allowing bills to affect it).

A median income earner starting employment at 25 and buying a 4 room flat BTO in a non mature estate at age 30, will no doubt hit the FRS by 55 even without any top ups or transfers.

Only a reduction of income, retrenchment for more than 1 year, or significant increase in the rate of FRS growth year on year will affect this scenario. (I did an exercise on potential pitfalls and scenarios for CPF and FRS)

And even then! There's still the option to do a property charge/pledge. And BRS is much easier to hit.