A tough AKA Killer Gm like myself for example is the nightmare of holy trinity mindset players. This is because a good DM if running smart enemies will use things like assassination to remove anything so obvious as a healer. First time the so called healer undoes hard earned injuries on the party that cleric is marked for death by the PTB they dare to face.
Likewise a properly played cleric wont offer heals to those not of his faith, without forcing you t0o convert to their faith anyways. THis is a key aspect of role playing a cleric few who did ever embraced. they felt uncomfortable trying to push their religion even a fantasy one onto their fellow players. First time a dwarf cleric tried to heal an elf player he lost all his powers until he killed that elf. His dwarven gawd who was lawful good was that offended by abusing his powers.
Finally a good DM doesnt balk at high lvl adventure, he embraces it as the time he can finally wipe out the party in a heroic fashion without any guilt.
I cut my end game DM teeth in 2nd ed on a lovely little module called the apocalypse stone. a story with a plot that tricks the party into triggering the end of the world, and NO WAY to stop it. With some fun suggestions at the end in how to run a post apocalyptic fantasy campaign in which the party now wanders the land as cursed death knights much like the infamous lord soth of krynn.
A dm who allows players easy access to healing magic is about as opposite of tough as wet tissue imo. If the party is badly injured and you for the sake of plot need them to be healed that is when the wandering NPC pacifist healer comes into play to offer some speedy mending of their war torn bodies. But in general a skilled DM wants his party badly injured most of the time to increase the sense of being at risk.
Ive been pretty much only a DM/GM now for close to 20 years. Ive DMed for 1st,2nd, 3E and 3E+ as well as Alternity and countless variations of D20 including my own amalgamated D20 that brings together the best of the various D20 systems.
Healing is the ONE thing that smart DMs always limited and hold tight reigns on if they want to keep the party off balance and feeling like death is just around the next bend in the road.