...or will there simply be insufficient technology available to allow the stealth meter to begin to approach the detection meter?
There will be techs sufficient to make the stealth meter approach the detection meter, although equalling or surpassing it will be very difficult, and would be analagous to attacking spearmen with modern infantry in Civ... or real-life.
It seems more intuitive to design around the assumption that both meters will progress hand in hand.
It seemed / seems simple / intuitive to have each notch of either meter provide benefits. If a ship can be seen at any distance when stealth and detection are equal, then there's no benefit to having more detection than an enemy's stealth.
As well, if there's a smooth transition between equal meters, which means detection at any distance, to slightly higher stealth giving undetectability only at very far distances, then that makes stealth essentially useless unless the stealth meter is overwhelmingly larger than the opponent's detection meter, which would be very difficult to make possible.
I suppose there could be some exponential dependence of detection distance on stealth difference, so that no difference gives visibility at any distance, a 3 point stealth advantage gives undetectability at moderate distances, and a 10 point stealth advantage gives total undetectability, but that makes things quite complicated compared to the current system, in which the distance at which something can be seen is proportional to the difference between the detection and stealth meters. Incrementing either gives the owner an equal, and hopefully clear and understandable, benefit with the current system. They can both progress at the same rate, with a net result of no change in detection distance, but just need to be offset in value so that the detection meter is always higher by some suitable amount.
When was this design decision made?
I don't remember when exactly... (Do you want a date?) It was during the stealth and detection design discussions, which may or may not have been conducted on the forums.