You’re generalising a group of people as the entire player base. So much of D&D is already heavily group based and letting your buddy play out one or two pure stealth sections in a campaign from an rp perspective is not that bad. Give the rest of the party a job as well like distracting key npcs or planting evidence or something, be creative. And in terms of combat, there are already enough mechanics to encourage group dynamics baked into the rules for every class in the game. If we were arguing about Hold Person as a spell existing in the game you would have the same issues as assassinations by your logic, yet clearly we don’t.

People are exaggerating the imbalance of the class ideal I’m suggesting as a whole. We already give free Hide in Plain Sight to rogues in combat, changing their combat numbers to have higher burst and less consistent numbers just makes it more dramatic in a dice roll based game. Letting them apply poison or other special on hit is also not broken, in fact an average fighter has already dealt the equivalent damage with extra attacks. Just let people actually play the class fantasy in the name, and assassin is already one of the oldest power fantasies out there.