It used to be either everything or nothing, or just sheer skill.
1. If you press E2, and the Q, if the enemy dodges, not only did you do no damage, you were vulnerable to enemy damage.
2. If you were good enough and you see that your E2 will miss, you wouldn't cast the Q in thr first place, saving you mana and more healthy trades.
3. If you hit both, nice, the trade is yours to control, unless you didn't see that a gank was coming and now you have no escape or Q slow.
Now, Sylas feels beyond annoying to play, I feel like the "all or nothing" playstyle really fit him, and removing that mechanic removes a lot of the skill expression from the champion. Now it just feels like "max E to spam it once it's on a 6 second cooldown". The Q detonation damage buff doesn't make sense, as a person with any brain cells is capable of dodging it. W buff to its damage and healing decreases Sylas's identity as a mage fighter, and instead forces him to build full ap to be another generic assassin mage.