I don't even see the argument here.
If a ball is fumbled and goes through the end zone, it should absolutely not be a turnover. IMO, you need to recover the damn ball to get the ball. It shouldn't matter if a guy is reaching for the end zone, or has it knocked out at the 10 and it rolls through the end zone.
If you fumble in the field of play and it goes out of bounds, the ball goes back to the spot of the fumble (whereas if you recover it, you get it at the spot of the recovery. That's the current rule on every play, except the end zone, with the exception of the last 2 minutes or on fourth down (where the only person who can advance it is the person who fumbled it, otherwise, all of those last second lateral plays would also be forward fumble plays).
It's such a simple solution that I'm shocked it hasn't happened already. If you strip the ball, you need to recover it to get the ball. If you fumble it through the end zone, you get the ball back at the point of the fumble.
If you want to penalize the offense for fumbling it for reaching over the goal line, use the 4th down/2 minute rule. If you fumble a ball forward, and it's recovered by a teammate in the end zone, the ball goes back to the spot of the fumble. If you want a TD on that play, then the guy who fumbled has to recover it.