I didn't say lower the DC. In fact, my suggestion would be the opposite. If you want players to invest in Charisma and Diplomacy more, raise the DC's.
Which I do at the table. I don't disallow a player from roleplaying just make the chances of auccess harder.
For me, roleplay is not something to reward with a bonus, it's the minimum requirement to engage at the table. I'm not a tyrant about it, but rather something I work with the player on and suit to their style of play.
I reward them with a bonus as well. Nor am I tyrant about it either yet at the same time their should be consequences for building a character with low cha and int. As a DM one should not bend the rules for low attributes since it penalizes those who invest the point in those attributes.
The major premise for a game system to work: to effect change, you must use the mechanics of the rules to do so.
Low attributes are still low attrbutes. They are tied into the system for skill rolls. Low con and str mean you hit less for less damage and can take getting hit less. Their is a reason why the system gives negatives to low attributes. If this was a rpg like Fate or one where attributes were not so tied into the system then you can have a character that is not as smart or charismatic while still bein effective. Imo this is not the system for it. Not without houserules or giving free passes to those with low attributes while screwing over those who invest the proper points in those attributes/
By "effect change" I mean something that matters. Sure, tying your shoes 'changes' them from untied to tied, but unless the success or failure is consequential, it doesn't matter. I'm talking about more important changes. Getting an NPC to do a favor. Making an orc dead. Finding a secret door. All these things require a roll.
Again what is being forgotten is that those who take low attributes should not be given a free pass on social situations of any kind. It matters not if it is a important situation or not. Want to take a cha or 7 or less go for it. I expect you to roleplay it. If not I have to start allowing a player using the Bard to hit and do as much damage as someone with a low cha.
Responsability for a player having fun is on both the player and the DM. More on the player because I run and design the game they are playing in. Not the character. Player want to hit and interact well in social situations then build the right character. I refuse to take any responsability for poor player character design. I don't force players to build characters a certain way. Yet neither am I forced to cater to a person who wants to have high con and str at the expense of three attributes of 7 or lower.