Naturally some GK’s have played different roles and been good at younger levels and could have gone some way, but there is a reason why they became goalkeepers. Nobody starts out to become a goalkeeper, which kind of answers the original question. On a general note, a gang of goalkeepers would probably reach the level of a lot of members on this forum. In other words, local teams and the format where you train 2-3 times a week and play a game, regional leagues or whatever it is called in England. They would be absolutely without a chance against Conference and Non-league teams, I don’t know the difference. Crawley showed us today the tiny difference between a motivated, shit team vs. a high level team not giving a shit.. the margins is often in intensity. The difference between a Conference-player and a highly paid grafter in the Championship can genuinely be the mental side.
A GK and an outfield player, it’s two different sports entirely. Even we could stand there, collect the ball and kick it out or to the nearest players with years of practice for the stress, but we could never ever play full-back. We couldn’t become goalkeepers either for that matter, but first and foremost focusing on the «outfield» aspect to a GK to make a point. No way in hell is Alisson or Ederson better than me at free-kicks or creating goals, and it’s not like I played at a very high level.
As for the fitness level, it wouldn’t help them either. Being top level football-fit is incredibly hard, the goalkeepers aren’t doing the same practice, they aren’t as fit as let’s say the full-backs, not even close at all. Sure they have good 02-levels and would run a fast mile, but that’s not the same as doing sprints, stops, sprints, get beaten in duels, sprinting again... all the time jogging/running, passing, tackling, shooting between the sprints. Usain Bolt would be wrecked after 15 minutes of a Premier League-game so to speak, different sports. Imagine Gabor Kiraly doing 60 yard returns in his sweatpants in the 75th minute - not that long ago he played in the Premier League.
Last but not least, most goalkeepers are lanky fellas. They would struggle to field a team due to constant injuries to knees, joints, ligaments, backs and all the crap that follows really tall people, speaking from experience.