But you have to realize the opportunity cost of using either one of those Pokemon. Gigalith does not really offer much to a team that something like say Golem can. Rampardos could be using a more offensive set with an item like Choice Band, but Rampardos is also not that great of a Pokemon, especially if it wants to get rid of a moveslot for Stealth Rock. If you want a powerful Stealth Rock, you could always use something like Golurk, whose Stealth Rock's power is comparable to Gigalith at 12.4%.
Sure, they could become more viable after this change... but does that mean they are good enough to be worthy of using them on every team? Probably not.
Well Stealth Rock is already a confusing move as is, with rapid spin being there to get it away. I find it simple to just spin it away, and if you're only doing 8.5% or even 10% damage on pokemon that take neutral damage from it, it's not really worth it. I want to be able to kill common switching pokemon as fast as possible, and the loss of stealth rock doing 12.5% every time. I need as much damage per switch-in as possible.