Again I'd like to use Kid Icarus as an example. The enemies can be pretty difficult to overcome without taking a lot of damage. That is in the case that you are trying to kill them as fast as possible. But because the enemies have so much more HP and do so much more damage, I find myself just running away and shooting lasers most of the time. This let's me avoid taking much damage while defeating the enemy. But it's a slow and repetitive process. In other words, it's long and boring.Here, you're making the assumption that it's no big deal to avoid taking damage. I've been assuming that the enemy is more difficult than that, or that the average player isn't an expert at the game. I've been assuming that the player is probably going to take damage in the base encounter, so an enhanced encounter increases that probability.
Increasing enemy HP will make the time required to kill an enemy increase, which also increases the time it can hurt you, but if the enemy isn't trying any harder than usually to hit the player, it'll be possible to avoid a 10000HP Super Hard enemy with the same ease as a 5000HP Hard enemy.
I'm not advocating longevity of its own accord, I'm advocating amount of challenge between safe zones or checkpoints. Consider, for example, how much harder Super Mario 3 would have been if there were no checkpoints, or if the levels were twice as long. It wouldn't merely be watching a cutscene to fill time or droning on in some battle, it would be increasing the opportunity for you to die because there are twice as many pits and obstacles between safety. In the same way, on a smaller scale, changing the HP on an enemy can increase the opportunity for you to take damage. It might not, depending on the enemy and the player's technique, but there are definitely places where it can.
And Implying that you are trying to create a fun game, this just doesn't cut it.