There has always been a limit on the number of overs that a bowler can bowl in ODIs/List A matches. I don't ever recall that being seriously questioned, but why not? There's no limit on how long a batsman can bat for, so why should there be a limit on a bowler?
I suspect the reason the rule originated to prevent the likes of Derek Underwood and Tom Cartwright bowling all afternoon and tying batsmen down in the days when the limited overs stuff began - now batting technique has had almost half a century to develop isn't the rule obsolete? and wouldn't these matches be much more interesting if bowlers could bowl as many overs as their captains wanted them to?