1. Instead of wasting 400g on tear you could buy an amplifying tome which would increase your early game duelling power.

2. Tear items are really **** on Kassadin and makes him very team reliant since he will do less damage than if he just went full AP

3. You have to wait to stack tear for your tear item to become useful when if you buy shadow flame you can start stomping as soon as you purchase it

4. You don't need as much mana as you think. Just don't spam R to get around the map like an ape and you will almost never run out of mana.

5. The extra CD that Archangels gives you is irrelevant since when you buy cosmic drive it makes up for it and gives you 105 ap in team fights.

6. Tear item takes up space in your inventory for a useful item.

7. Kassadins R mana conversion into AP is worse than just buying AP.

8. Makes it a lot harder to 1v9 since you can't just run up and one-shot the entire enemy team.

Play safe until level 6 and after that, you can usually out-trade any matchup midlane.
Don't use your R to get around or else you will have very little mana to fight with.
Team Fights are really easy on No Tear Kassadin since you can one-shot everyone with little effort. Just play around your W if you somehow run out of mana during the fight.

/mute all
Trust me if you are below D3 you can carry every game just mute your team and play like a psycho.

This is probably the best wave management guide out there so watch it and try to retain the info. Most games can be won with good macro not micro.
https://www.youtube.com/watch?v=p9ZUxaCpvjc