I'm going to take a stab at trying to explain why I think it would be a bad thing for the game to present the option to decrease ability in one pillar to excel at another.
I prefer to have PCs who can contribute equally but differently in all situations. I like proactive, invested players, who when presented a situation sees multiple options laid out before them. Those options shouldn't be driven by their character's mechanics but by the fiction of the game. A character who has given up a great deal of combat prowess is going to want to avoid combat. A character who has given up a great deal of exploration options will not care about exploration. A character who has given up the ability to take part in social interaction won't be as interested in intrigue. So by giving hte option to remove combat prowess in favor of, say, social interaction, to my mind, is taking away useful options from the player. Suddenly you aren't siloing abilities, you're siloing PCs.
We're going in circles now, as your viewpoint was already presented by others, and addressed. Inherent in this view is taking the concept to the extreme, and claiming "a great deal" of combat prowess is given up. We're not talking about that. Nobody is saying you should be able to trade all your combat abilities, or even most of them, away for a non-combat one. It's not an argument of "let's set this precedent with X, and then also spread that same concept to Y and Z too".
We're talking specifics here, not generalities. Specifically, we're talking about swapping sneak attack for a more non-combat ability. And as I've already addressed, the rogue is not crippled in combat by losing the sneak attack ability.
You're NEVER going to achieve perfect equality in all things by all classes. Nor is that a desirable thing to constantly move towards, because it tends to result in all the classes feeling roughly the same, which was one major complaint with 4e (whether you agree with that complaint or not). Based on feedback given to WOTC in their surveys, a majority of people want characters that have a wide variation in their abilities and focuses, and balance between the classes in all things is not the highest priority. It's OK if one character is better at social encounters than another, one is better at combat over great distances than another, one is better at combat at short distances than another, one is good at capers that require a lot of sneaking around and scouting than another, etc.. It's not an undesirable thing that the classes are not equally balanced in all these things.
Beyond that, I want players who are invested and excited by all kinds of different aspects of gameplay.
This is up to the DM, not what's written on character sheets.
When the PCs come into the den of thieves and have to convince the guildmaster to help them, I don't want gameplay to suddenly be driven by the guy who forewent Sneak Attack for Improved Parlay or whatever ability replaces it.
Why the heck not, it's what his character does best! Just like it's what the fighter does best when in melee combat, or what the wizard does best against groups of creatures, or what the archer does best at long distance combat, etc...
I want the group as a whole to come together, for everyone to want to interact here.
So engage them as a DM. That's your job, not the job of the rules. You're basically arguing everyone should be using their character sheets to seek out options for that kind of encounter, instead of just telling you what they do and interacting with the DM and other players. That's, in my experience, not the most conducive method of engaging players (even the ones good at that task).
That means I want every PC to have a relevant interaction ability.
Well, they won't. In no version of the game did they, and clearly in this version they won't either. Some people will be better at some things than others, as it should be, as it's always been. And if the Wizard has time to re-memorize spells or buy a scroll of what he needs, he will flip his abilities in a single day, trading a combat spell for a non-combat one. There is no way to make everyone as good at everything as everyone else, without the game becoming incredibly boring.
This is especially true if this encounter is the climax of the adventure or session, just as if it had been a fight with a BBEG.
So whatever option you want to give the PC in exchange for Sneak Attack? I want that to be siloed out. I want all rogues to have access to that as part of their interaction pillar. Then, they can change that out for something different way of interacting. Options are great. Removing options isn't great, and I don't think it should be encouraged or really even allowed.
Well, by saying you don't want it allowed, you're saying you want your playstyle to be forced on everyone. I don't get that. Not everyone plays the same as you, or has the same style as you, or desires this equality that you seem to value very highly. They should have at least optional rules in a future rules module/supplement that lets them adapt the game to the way they want to play it.
But, don't force character to choose which pillar to specialize in, which is basically what would happen if you could trade one pillar for another. If someone wants their PC to be a smooth talker, they shouldn't have to give up their ability to be an equal in combat. That's not something I want to see in D&D.
So I guess you want ability modifiers to be removed from everything as well? Because they have to given up an ability score somewhere to have a high charisma, which means they traded being good at persuasion-abilities for being bad at something else. And of course the wizards could no longer switch spells in a day.
This goal you're seeking is unreachable, and not desirable to begin with. People LIKE their character being good at something nobody else is good at. This is the whole friggen purpose of the class system. D&D isn't a class-less system. There are plenty of other games out there much better suited to the goals you're stating, but in no way is D&D one of those, because it's all about unique classes and abilities not shared by all the other PCs.
