2strokes are being replaced because they are dirty engines. In the eyes of the EPA. Stihl can have some dirty engines but they have to have so many more clean (4mix) engines. All the 2 strokes won't go away this year. It will take a little while. And Stihl is the leader in this category. Every other company will follow suit after a while
But Stihl dont always make their own engines.
If you strip down certain engines on a Stihl product they use different engine manufacturers.
1 name is Tecomec (Italian company owned by Emak who make Efco or Oleo-mac product) who produce 2 stroke engines for different manufacturers.
Another is a German company called Mal. Stihl like to buy off of German companys.
Basically OEM's buy a product line for a certain time and the engine manufacturer cannot give that line to any other OEM. The change in line up could be as simple as a different carb (Zama for Stihl and Walbro for others)
Another OEM buys another line with the same engine and again a different carb and so on.
Lots of OEM are using the same engine but set up differently.
Stihl were always good at making Chainsaws but struggled on the engine side.