Here is how it works. You reserve a concrete stone by paying a small fee. The stone has enough room for a tiny capsule containing either ashes or small memorabilia. When you die, your stone is added to the pyramid.
To reach 400 meters high, you need over a 100 million stones. So capacity is not a real big issue. Then again, if this becomes a popular way of being remembered and billions upon billions of people die over the next few hundred years...well...at least its way more efficient to incinerate the body and save a small bit of ashes for a small 90 X 90 X 60 cm brick than it is to give each person a big coffin and a grave.
I would want my epitaph (ever brick gets a small epitaph) to be clad in bronze so when far future archeologists are sifting through a 2000ft tall artificial mountain, they will find my little name plate and be like "So the Pharaoh's spoke English!". I am pretty sure bronze outlasts concrete when it comes to being broken down by nature.