Hey Slovakattack,
You certainly bring up legitimate concerns with the buffs.
I don't think that I agree with the fact that giving him substantial burst will make him an unhealthy champion, as long as he has to close into melee to achieve its full potential (even for just a second or 2). Removing the Silence has given us a lot of room to move now that his opponents are not completely locked out of reacting to him.
At the end of the day, Kassadin's burst is still not top-of-the-line in comparison to other assassins / mages, his base damages are among the lowest in class and his AP ratios are sizeable but not insane. A mobile, high burst, but squishy Kassadin with a hit and run combat style is something I believe is core to many Kassadin players. The buff to the W is simply to make it compare more favorably relative to Q / E and give him more reward when he arrives at his target.
We ran into some difficulties when creating a sustained damage Kassadin combat style, primarily due to the Mana itemization that exists in the game. Mana items are inherently very tanky, specifically they have a ton of easy access to Armor. Combine this with the focus we want to put on anti-magic in Kassadin's kit, this becomes a bit problematic because he often quickly becomes extremely resilient to both magic (built into his kit) and physical (Mana itemization) damage. Whenever we give a champion both Tankiness and Mobility, they need to have some pretty severe tradeoffs elsewhere on the kit or they become overbearing to play against. For Kassadin, he has to make real tradeoffs between Tankiness and Damage which is something we need to preserve.
Ultimately, I'd love for both Burst damage Kassadin and Sustained damage Kassadin to have places in the game, but to do so we would need to ensure each has its own strengths and drawbacks. With the current Mana itemization in place, I don't think it's plausible to make a sustained Melee DPS version healthy for the game. Currently, Kassadin can opt-in to a riskier AP focused build where he has significant damage and mobility but is ultimately very squishy, or a Mana-Tank build which is tankier and more controlling at the cost of damage output.
Hope this helps explain things a bit.