No Driver is replaceable, each champion has their own impact on the sport. That said is he the greatest of all time? Maybe but it's not a lock. There are many ways to judge the greatest F1 driver of all time, pure statistics, winning against impossible odds, etc. One could argue that some of Michael's success is being at the right place (or team) at the right time. What would Senna had accomplished with the same circumstances if his life had not been tragically cut short? Or what of Gilles Villenueve? In his own words, he only cared about winning races, not championships... for pure talent alone he might be the best in my book, at least in the last 30 years.