In most RTS games, static defenses have hard counters by some sort of siege unit that is useless at most other things.
Supreme Commander FA for instance, has point defense which out ranges all other T1 units except for the mobile artillery which is the slowest, lowest hp, and its projectile takes the longest time to reach its target. It is for most purposes useless at unit on unit combat, but when destroying the T1 point defense (which is very good at killing t1 units), it out-ranges it and will kill it after a few shots. there still is a defenders advantage as these siege units are relatively expensive but defenses are there to deter and buy time, there is no perfect defense. defense revolves around having reserve units to rally to the defense, not a tower defense approach.
that game also raises the stakes by having a concept called reclaim that allows you to recycle destroyed units/structures resources and creates a meta game of controlling past battlefields, for the reclaim.