I'd have to disagree about removing SnS, from a pure gaming point of view this is one tactic out of many available for use. Also not out of the realms of reality whereby returning artillery fire would be delayed as they were repositioned/targeted.
As a game I agree we cannot slow it down any further and it is already quite possible to spend weeks fighting over single city.
There are ways to counter pretty much every tactic in Supremacy, if you take away SnS you give those who use tank armies a greater advantage - do we then do something to hobble the tanks to even it out again?
The complaint about SnS players being 'too active' is quite lame - every game has players with different activity levels and those who only play once a day are always going to be at a disadvantage however what gaming company is going to punish those who play long hours? If someone is willing to put on the hours and check often then they deserve to be rewarded with a win IMO.
Instead of trying to remove a feature you can't/won't use why not find a way to counter it that your playing style allows. I hate coming up against a blitzkrieg player however I hunker down and deal with it.
Each weapon type has it's pros & cons it's up to the individual to learn how to use them to their advantage.
You could equally say planes are overpowered and as yet no real defence against them (other than to fool the game into targeting the wrong army while you flee), not even the likes of anti-aircraft guns that were available in WW1 - before we start changing what we have why not add the things still missing.
Some things just wouldn't fit well into a game, as Narmer already mentioned to include trenches & bunkers would not add much value but it would slow everything down, but I think there is still room for some things and since planes were introduced we have been missing the obvious counter weapon for them.