I doubt you tested ledge options vs ledge options thoroughly. It pretty much translates to whoever goes to the ledge first will lose because your invincibility wears off first. There are a number of extra things that many characters can do that aren't possible on other ledges.
All of a sudden people care about ledge games being broken? If Pipes is banned, MK should be banned. And none of that "but people don't main stages" junk.
Grabbing the edge first is not a death sentence for planking. That would be as naive as saying Norfair doesn't allow planking because you can stand on a lower platform and be right next to them when they drop off.
Let's assume that it is an incredible disadvantage to grab the edge first however. I'm G&W, and you're someone else. I'm planking, you come along and grab the other edge a bit after I do. Now what? I'm supposedly at a strong disadvantage, even though I get to move off the edge first (still with the same invincibility as if noone was there) and the longer it took for you to grab the edge the bigger of a window I have to get away from you. If you take even longer, somebody can just complete a full loop (drop down, DJ/Upb back) before you can really cover both ledge areas with a hitbox at the specified window. What's the point of getting invincibility after I do, if it will also slow down your attempts at stopping me? It's already hard enough to time an attack that will stop them, or grab their edge at the key moment during regular planking when you have all the time in the world. The last thing you need is crunching that window down for the sake of being invincible. Obtaining invincibility tends to only help out people who are good at/against planking in the first place. Falco with invincibility is still butt at stopping it, where as Marth/G&W/MK/Pika/someone established can certainly improve from that.
If you grab the edge at the exact same time that I do, it tends to boil down to whoever has the best ledge game will still prevail. Other options will also come into play, like going back on stage purposefully if you think they can't punish you for choosing that either cause they have a bad "get back on" game or because your character tends to stuff a lot of options for people trying to get off the ledge and back onto the ground. Marth, G&W, MK, etc being the main culprits in that regard vs Pika/friends. It's a smaller version of the MK conundrum , where stopping his planking by taking the edge from him also opens the door for him to go onstage and in the near future return back to the edge and continue without harm.
If you want to put a stronger LGL on that, go for it. I'm not an advocate for making stage specific/entailing rules to try and redeem them, but it's true for a lot of stages that you can make them legal in a competitive setting IF you are willing to go completely out of your way to make rules against abusing any broken facet of the stage regardless of how enforceable the rules are. I would think it odd though that people would complain about a surgical rule like LGL, but be entirely in favor of slapping on stage specific LGL or "don't circle camp/run away/abuse X facet" kind of rules.