Is Kassadin healthy

A full AD comp is still a massive drawback. Your Braum simply wasn't the smartest cookie. :^)

Most old champions have free stats like that somewhere in their kits. Hell, bet you didn't know that pre-rework Urgot's passive was basically "enemies you damage deal 15% less damage." Anything he hit would deal 15% less damage until he stopped hitting them. As a result, his damage was incredibly low and cost a ton of mana to get off as compensation so that he wouldn't be able to 1v1 everything in the game with no contest. And also notice that a lot of the champions who've been reworked lately have had those free stats shaved off their kits, and that many champions have had their passives alone reworked to get rid of these. Take a look at Zilean. Original Zilean's passive was just your allies gain 8% more exp. Global. Everyone on the map who was allies with Zilean gained bonus exp from nothing. Janna gave everyone movement speed, TF gave everyone first crit, then gold, and now only himself gold. I believe Riot has said that Kassadin is on their list of champions to rework, so when that comes around, I can guarantee you that his free stats just for existing passive will be gone.

Also should be noted that Kassadin used to be a LOT less healthy. His W would actually take the mana it gives him back from his opponent. His ultimate cost a lot less mana for overuse and had a much longer blink range. He's in a much better spot now than he was back then. Presence of Mind Kassadin still OP though.

Free stats are generally not the best design, but a champion's gameplay health needs to be evaluated as a whole, rather than simply based on a single component. A champion with 1700 health and 15% magic damage resistance may be healthier than a champion with just 2000 health, for example: both have exactly the same effective health versus magic, but the former champion is more vulnerable to physical and true damage, and so has more counters than the champion with just health. Base stats are "free stats" as well, but are necessary for a champion to function, so if a particular champion needs a bit more special tuning, a stat passive can also help in that kind of situation.

As for the question in the title, I'm personally not entirely sure. Kassadin is certainly healthier than he's ever been, but I think he has some ways to go before he can be truly perfect. One problem I see with him is that, despite being an assassin with the perfect kit for dodging skillshots, much of Kassadin's anti-mage power comes from him facetanking magic damage, namely with his passive and Q shield. Additionally, because he lacks long cooldowns on his kit, his power's practically always-on, which can make him difficult to get under control when he eventually starts scaling. His E is technically on a longer cooldown than is indicated, but is also gated very awkwardly, and could stand to be improved. His Q also fundamentally lacks interactivity, and is stuck halfway between being a nuke and an accurate interrupt. If I were to change him, I'd take out his more face-tanky aspects and instead make him more focused on nullifying and dodging spells entirely, with a big, lower-uptime damage ability to reward him for doing so enough times.