Another way to look at it
When you get right down to it, the highback forward lean is designed to force you to flex your ankles, which will in turn generally force a more flexed knee as well. When snowboarding, we almost never want to be opening (extending) our ankle joints.
When initiating a heelside turn, we want to pull up on our toes while keeping our knees bent (this will flex the ankle joint). Many riders are lazy here and just rest against the highback and relax their ankle, this makes for a lower edge angle and more leaning of the upper body (we call this inclination). This leads to less performance, and less balance as the upper body moves away from the board. On the other hand, if we have the highbacks dialed in some, we can use the angles of our lower body to create a high edge angle while keeping the upper body over the board (we call this angulation). This makes for more performance with less movement, and more balance because our upper body stays over the board.
When initiating toeside turns we want to also flex our ankles, especially our front ankle. To test this (at low speed please), try extending your ankle when starting a turn (like standing on our tippy toes), how does it feel? Tiring, and wobbly most likely. Now try flexing your knee into the shin of your boot, allowing your ankle to flex with it. you will feel more power, and stability. For all the reasons that apply on heelside (inclination vs angulation).
Lastly all of these improvements will reduce the movement required edge to edge making you smoother and faster changing edges.
As a summary, can you ride perfectly without forward lean? of course! is it easier to show these proper and efficient movements with some forward lean, definitely! It will come down to personal preference how much forward lean you will like! I suggest playing with it, seeing how it feels, and seeing what you like. I suggest small changes to it and ride for several runs before deciding whether you like it or hate it... Either way, have fun and keep shredding!