The relevant points in the DMG are pretty simple. If multiple PCs participate in a conversation, the players decide who makes the charisma check. It encourages things like insight during the conversation to figure out what the NPC is thinking. Adding to the conversation grants advantage to the person that makes the check. It specifically states "Create situations where characters who might not otherwise be engaged with a social interaction have to do at least some of the talking." Along with "If a couple of players are dominating the conversation, take a moment now and then to involve the others." A conversation may involve multiple rolls, etc..
If you follow the chart on reactions, during a conversation the worst thing is that you get is "The creature opposes the adventurers’ actions and might take risks to do so." if you get a result of less than 10 with a creature that was already hostile at the start of the social encounter. Last, but not least, bards are less capable in combat because they are supposed to be the skill monkey outside of combat. That doesn't mean the balance is perfect, but if you follow what is actually written in the DMG you still include everyone in social encounters on a regular basis. If one person is dominating every social encounter, the DM is not following the advice in the DMG.
Could the DMG be better? Absolutely. Are people that have 1 person dominate the social aspects of the game while everyone else is mute following the guidance we do have? Absolutely not.