lichmaster
Hero
In a system like D&D, IMO the simplest and most effective solution is to ignore XP and go for milestones, but that's a taboo for most people.To fix survival, you need to fix the adventuring day. To fix the adventuring day, you need to fix resources per day and XP to next level. That would require an overhaul so extensive that no class or player supplement would go untouched.
IMO XP make sense in level-less (and probably class-less) games, which also tend to have more unified systems instead of specific designs for each class.
Resources per day are more tricky to change for the same reason: a unified system needs to be tweaked in one place only and all players will experience the same effect (assuming they use that specific resource at all), while a in a heavily class-based design this can be very hard and time consuming, because all possible interactions should be considered (and because even if classes have access to the same resource, they may not share the same basic progression)
Case in point: if one wanted to change the number of spell slots, with a unified system one would simply change that table and the effect would be the same for all classes that use spell slots. With the current design, where each class has it's own spellcasting progression table (even if most of them are identical!), such a change needs much more work.