Here we go again…default crossplay on

Why turning it on by default to begin with?

That's an extremely easy question to answer. A bigger player pool means that it's much faster to matchmake and in addition to that, it can matchmake players into a larger variety of game modes and maps.

However a lot of casual new players do not. They boot up the game start matchmaking and get beamed.

Casual players get beamed because they are casual players. They would get beamed regardless of whether cross play is on or not. In the vast majority of cases, it's not about whether you're using mouse and keyboard or a controller. It's about whether your knowledge of the game leads you to make the correct decisions. Short-term decisions like positioning, crosshair placement and target prioritization. Tactical decisions about which objectives to attack, which routes to take and when to engage or disengage.

Don't get me wrong though, your ability to aim is certainly still important, but that's where aim assist for controllers kicks in. Controller players only have about half the recoil to compensate for as well. 

It will negatively impact player retention. It especially becomes an issue in low populated areas, increasing lobby waiting time later in the game’s lifecycle.

It's the complete opposite actually. Generally cross play boosts player retention because the game will maintain an active player base for longer. The idea that people quit out of frustration simply isn't true. Having said that, there's one key component to making cross play work and that's making sure the console experience is properly balanced against the PC experience. It should also always remain optional.

Besides, I disagree with the idea that casuals can't find an option in the settings. Obviously they shouldn't bury the setting in layers of sub-menus. If it's easily accessible, there should be zero issues with leaving it on by default.