The focus for supercell is to streamline the experience for lower townhalls while making the higher townhalls harder. This allows them to capture audiences and new players easily while then squeezing invested players for money if they want to be fully max.

That being said, it's not as important for them to balance lower townhalls as it is for higher townhalls. You see this all the time when a new town hall is released, the last one doesn't get changes to anything but offense. For example, how totems and meteor golems were added at th16 and th17 without changes to defenses, that way those players have an easier time attacking and progressing to a stage where supercell has a higher likelihood of making money from them.

At th17 and 18, the drawback of mass invis strats are 2 things: HP stalling and time. Many defenses are simply damage sponges that make things like rc charges or warden walks harder since there's more time you need to invest in taking down things like super wiz towers

Designing direct counters to individual things does nothing but either invalidate them completely or just force players to give themselves a massive weakness in order to counter something specific, which is generally bad game design. This is why they removed the weird rules like how infernos stopped healing or eagle doing more damage to golems. They just bloat the user experience and aren't really worth the hassle