This is the whole concept of minstrel{,-ht}, the rate control algorithm in charge of all mac80211 based drivers (aside from those doing rate control in firmware). Rates are carefully stepped up under load, until excessive packet drops/ retransmissions are encountered, which is when it will slightly back off again - if there is no traffic flow, it ramps down to the minimum rate (for a mobile device (but also changing interference situations) this is essential to remain connected even when the reception worsens). While it will take a (short) while to ramp up again, there isn't really any hurt coming from it (after all, established data rates don't matter if you're not transmitting/ receiving anyways) - and it doesn't require a lot of (background) traffic to remain on the higher end.
There have been quite a few attempts of writing rate control algorithms in the past, both before- and after minstrel (e.g. ath9k had its own at the beginning) - none of them really were a competition. Even today's in-firmware rate control algorithms (e.g. intel) are often more of a pain, than a blessing.