Why is it better to be a caster and spend slots doing minor healing, rather than being another martial and reducing the damage taken by everyone else by taking some yourself?
That's fine but that's not the issue I described.
The issue is that the frontliners in D&D 5E go down about 5x as often as any other PCs. That means the frontliners will be the ones subject to the system you're proposing.
On top of that healers (only) will have to spend spell slots - and it won't be minor healing - the day will be over if they let a frontliner get downed.
Whereas casters who can't heal? They'll just get to play exactly as normal, or very close to it.
Again I don’t really want to make this a martial caster debate.
Sure but the
practical reality is you're proposing a system that will essentially destroy the ability of martial characters to continue after they've been downed even once, on average, but will not do the same to casters. You proposed 1d6 level of Exhaustion for being downed once - that means there's a 66% chance they'll be made completely useless if they get downed once (5E exhaustion rules).
The frontliners are whoever are in combat at the time. If the fighter falls, then someone else is gonna be the frontliner. I don’t believe the traditional 10’ corridor with two up front and two in the rear will apply here.
Sure, but if your have two downed frontliners, you're inches away from a TPK in any version of 5E. The reality remains that unless you're proposing other rules changes, the real frontliners, the day-to-day ones, will be downed about 5x as often as others.
Though I disagree that exhaustion - particularly One D&D rules doesn’t affect casters. It reduces spell DC by one per level of exhaustion.
Are you planning on using 1D&D or 5E exhaustion? They're drastically different. 5E is the only one which offers the "chance of death" on a 1d6 roll, and you seemed to be keen on that. And 5E demonstrably doesn't impact casters much simply because they don't make rolls - if you added to it that it gave enemies Advantage on their saving throws (equivalent to the martials having disadvantage on their attacks), that might be a thing.
Again you’re misunderstanding the point of the thread. This is for a specific campaign - not changing the rules of D&D for all time.
Sure, but is the goal of that campaign intended to be that martial characters regularly get wrecked and healers need to pump them full of heals (seriously - it will need to be most rounds if they're taking damage because 5E heals are tiny), whereas non-healer casters (primarily Wizards, Warlocks, Sorcerers) will get to play as normal? Because it sounds like it wasn't.
What you're proposing is going to be very punishing for anyone playing a martial or martial-adjacent PC, but feel great for anyone playing a non-healer casters, because of three factors:
1) You're proposing shorter adventuring days - this favours Long Rest classes, particularly if these days don't involve Short Rests between every encounter. This would be an easy-ish fix at least - reduce a Short Rest to 10 minutes and cap it at 2/day. So that can be dealt with.
2) All the things you're suggesting doing, will primarily affect martial/martial-adjacent characters, secondarily affect healer casters, and barely even impact non-healers. That's not really debatable, it's just the consequences of the rules you're laying out. That's fine if that's intended, but one needs to be aware of the consequences.
3) The martials getting mangled on a down, plus the healer casters having to aggressively heal them means combat days will be even shorter. It would be simply suicidal/stupid/murderous to proceed when a PC has been downed. So Long Rest casters are going to be even more favoured, as 5MWDs will be far more common in this approach. For hopefully obvious reasons.