This post assumes devs and players alike consider being able to see the entire map with a single ship loaded with 30 sensors is a bad thing and not an intended game play mechanic. If that's not the case then there is no problem (other than people don't think it's a problem ).
There's a couple things I don't understand, maybe you guys can give some explanation that makes sense.
1) Why do sensors stack in the first place?
I get that the games ship building system works by stacking stuff, and that works for most things. The devs figured out that it doesn't work for everything, like repair and rapid fire modules. Why did they not think the same about sensors?
2) Why is making them not stack not the solution? It's hard not to say it without sounding like an ass, but it seems pretty simple and obvious to me. The devs have probably played other 4x games. Every other 4x game I've played didn't stack sensor range, they increase in range with tech level, the size stays the same and maybe they get a little more expensive. Want to see more of the map? Invest your research in sensors or build another ship. This mechanic works fine, changing it isn't going to revolutionize the genra. Stop trying silly things like increasing mass with range.
Alternatively, if people have such a stacking fetish, you could allow one of each tech level on a ship so people can still utilize a ships capacity to increase its sensor range, but it would have diminishing returns and ultimately (here's the key devs) a limit. Set by the the players sensor tech level, not the ships capacity.