You can say that it's not the point... but, oh, it IS ENTIRELY the point.
A player has to be ON THE FIELD in order to get snaps and be an integral part of the offense that we all would hope he can.
But.
A player has to NOT FUMBLE in order to gain the confidence of coaches and GET TO BE ON THE FIELD to any appreciable degree.
He has 3 preseason games to go. Imo, he can fumble once, and it might not make much of a difference. He cannot fumble twice w/o endangering his spot on the 53-man, and unless there's some contract thing that would intervene that I don't know about w/o researching it, his career as a Cowboy will be over before it ever started if he experiences 3 of those, and I don't care if it's after a carry, a reception or a kick/punt return. (For most players, we don't even imagine 3 turnovers in 3 games to be in the realm of possibility. Even for Tavon, it's highly unlikely. But he is one of those players that, indeed, it IS within the realm of possibility.)
And even if he makes the team as we all are wanting if not expecting to see, there will remain a short leash... that is, until he proves during the season EITHER he is comparable in some way to those upper tier WRs you cited (which again is actually going to YOUR point above that players can play well enough to be excused for some shortcoming in one statistical area), OR he has indeed turned the corner and is at least average in terms of ball security.
Summary: I am one of those who is confident that if Austin has ball insecurity problems to the degree here that he did with the Rams, his experience in Dallas will be not all that different than his previous experience with them.
OK, let's try this. Let's say Austin touches the ball (receiving and rushes) an average of six times a game. On his 48th touch (which would be in his eighth game), he fumbles. Do you cut him?
He fumbles again after 24 touches (4 games), do you cut him?
I get that he has a fumbling issue on punt returns, and I want to see him returning punts during the preseason to see if it's going to be a problem. I have no problem taking him off punt returns.
Your standards for receiving and rushing don't make sense, though. If he fumbles twice in his first game or fumbles multiple times over the first few games, then I'm with you. Other than last year, that hasn't been his history.
He really had a bad stretch from the last two games of the 2016 season, where he had three fumbles, and the first five games of 2017, where he also totaled three. If he does that, then yes, you certainly don't rely on him. In 2016, he didn't fumble until the seventh game, when he had two and didn't lose either. (He had had 40 offensive touches coming into that game; I'm not sure how many punts he had fielded.) The only fumble that he lost was in the last game of that season.
As I said, 2017 was worse (and his worst season overall handling the ball, which I'm sure is one reason he was traded). He lost fumbles in the first, third and fifth games. If he does that this year, I'll be leading the march to bench or cut him.