Yes, in the games I've been in the DM does provide for enemies attacking downed characters somtimes, depending on if it makes sense. They also get to suffer from area effects. So not even just stabilizing a downed character is enough. You need to get them to a reasonable number of HPs or else they just go right back down again the next round.
That's fair. Cleric and somewhat Paladin are the classes that excel at healing a lot with a single turn.
DnD 5E is more or less based around the idea that preventing damage is far more effective than healing it. The real value of a cleric isn't based on the healing they can do, although it's a plus to outright prevent a death for a downed character. It's the crowd control options and defensive buffs they offer.
Agreed. Which is party why other classes can easily replace a Cleric imo. Druids and Bards, and to a lesser extent Paladins, Wizards, and Sorcerers, also have CC & (de)buff options that can replace a Cleric.
The most important role in D&D imo is a tanky frontline fighter. A party without such
can work, but they'd need a lot of caution and tactics.
