Mid mages actually are fine, but the meta has changed to favor frequent bot lane roams for turret first blood. Try tweaking your build for more early roaming power. For example:
instead of

The game is basically decided in bot lane by the junglers and roaming mids deep diving. Vel, Vik, Lux, and Xerath are all extremely good on dives, but you have to be there for when bot lane goes full clowncar.
And honestly, the other thing you can do with some mages when you're really ahead is to pass along some of that gold value to your team. Like, if I have my damage core completed on Lux, I'll build an
because it translates my gold into turret-pushing power in the form of a team-wide 20% aspd boost. And it's dirt cheap, and the AP is (sadly) not much lower than most AP carry items these days.
When I play Zyra mid I build
as my only mana item because it's more than enough regen and it pays for itself over time. I'll upgrade that later to
to counter invisibility or to
+
to completely dominate vision.
Also, with as hard as bot lane gets camped now and as early as the first turret goes down, it's really not a bad idea to have a gold income item anyway. 9 games out of 10 your adc will be farming your CS in mid by 10-12 minutes anyway, and you'll really fall behind if you haven't snowballed by then.
The new meta has definitely been jarring and hard to accept. But the reality is that games are decided in bot lane by the jungler, and the mid laners with the highest influence will be strong roamers or have high utility. There's a reason why
play rate increased by some 40% while his win rate also increased a full 2% (now over 54% average) - turret execute and a big ult in bot lane while safely farming in mid.
This is also why you see some interesting changes to bot lane pick/win rates with huge spikes for both
and
. They're both very strong on dive and anti-dive.