Well, I’ve given several reasons:
1. Not gating the resolution of certain challenges behind magic encourages non-standard party composition;
2. Rewarding player choices: a character taking Expertise in Medicine tends to be sub-optimal. Having the occasional situation where it is clutch rewards player choices.
3. World-building: if the party retreats to their original village for healing, the aged village healer might have Expertise in healing (or might not). The aged village healer being a 9th level cleric strains verisimilitude.
4. Class balance: One of the main defenses for casters’ power level is that they trade power for versatility. A caster isn’t supposed to be as good at certain tasks as a specialist, but is more flexible at a variety of tasks. That isn’t the case for either Mummy Rot or the Clay Golem’s Slam. The caster is simply better than the specialist.
5. Expertise should feel special: Rogues get 4 Expertise slots, bards 2. Everyone else needs to trade a valuable feat for it. Unless you take Expertise in a skill with a combat application, it often doesn’t feel very special. Hence, challenges that require someone to be an expert in their field to even attempt.
6. Challenge: the justification for Mummy Rot or the Clay Golem’s Slam is supposed to be to challenge the players. But it’s not really a challenge, is it? It’s a luck-based mission: did you fail the saving throw (set deliberately low because of the deadliness of the effect)? Do you have the correct party composition to turn the encounter into a speed bump?
’s premise was that he was feeling somewhat burnt out by high levels of magic.
I’ll accept that there are certain challenges that can be overcome only with magic, but the onus is on you to make the case in favour of it.
“Why wouldn’t it be?” isn’t an argument; and “because it’s magic!” is a tautology.