In Unity's ECS they literally had to throw out every single thing from the OO game engine to get ECS working.
So it was like having two entirely different ways of building a game in the same game engine. Everything one knew about Unity was now gone and you had to learn an entirely new way of solving the same problems. Sure you can still do OO things like inherit systems or whatnot but it was nothing like the original unity.
Once they realized how big of a change this was they did come out with some hybrid approach where you can have OO objects converted into Entities. But really it was just a Band-Aid to the gaping wound they made of the engine.
So it was like having two entirely different ways of building a game in the same game engine. Everything one knew about Unity was now gone and you had to learn an entirely new way of solving the same problems. Sure you can still do OO things like inherit systems or whatnot but it was nothing like the original unity.
Once they realized how big of a change this was they did come out with some hybrid approach where you can have OO objects converted into Entities. But really it was just a Band-Aid to the gaping wound they made of the engine.