Static health may make mages more durable, but they still come nowhere close to tanks or fighters in terms of survivability. By contrast, if the marksman doesn't get bursted down, their life steal can allow them to heal from any lesser source of damage at no cost or down time. In addition to scaling into the highest damage in the game, marksmen thus end up also becoming de facto drain tanks, to a greater capacity than champions designed for that same function, like Swain, Vladimir or Warwick.
The thing is, this isn't good for marksmen either, because it's what's forcing mages and assassins to have burst that's either too high or too low on counterplay. Since the purpose of those classes is to take out the enemy back line, which includes the marksman, and that marksman can a) heal for hundreds of health every second, and b) deal hundreds of guaranteed damage a second to any target in range, if they can't burn through the ADC's ~3000 effective health within a couple of seconds, they will die and their work will be erased in the next few seconds that follow. Effectively, you end up needing to give these burst champions about 3000-4000 damage in a quick rotation that also allows them to either escape safely or operate from a safe distance, just so that they can have a chance of doing their job right. This feels fair to no one, least of all the marksman, but it's a necessary evil in an environment where their vulnerability to damage is so binary.
Taking this into account, the game would likely be in a much better state without life steal: marksmen are supposed to be naturally weak to burst and strong against DPS, and while the lack of life steal would give burst champions more leeway against them without needing ultra-reliable or overly strong damage, it would also not significantly hamper marksmen against other DPS champions, i.e. fighters and other marksmen: marksman vs. marksman fights would be a little less item-dependent, and fighters would still have trouble due to how they are naturally vulnerable to kiting. In fact, this would also be a good opportunity to take the burst out of of several Divers, such as Hecarim or Vi, as they wouldn't need it to hold up against the back line. Similarly, assassins and mages could afford to have more counterplay and unreliability injected into their own kits, as there are still quite a few outliers, e.g. Malzahar, Rengar, Syndra, etc. that aren't fun to play against, and/or have been bent out of shape after repeated balancing.
which of these poke mages were op?
: great range, fairly good cc, high burst with setup, ok burst/dps without setup.
: some decent ranged poke, high burst, also has mobility and hard cc with setup.
: good range, ok burst, good cc without setup, great cc with setup
: good range, great conditional range, decent burst (trap auto -> E + auto), good zoning cc, slight mobility/self peel.
