at level 18 mana is not a constrain for any champion, but at lvl 18 pretty much any champion can blow up another, damage in the game is the highest it's ever been (defense is also higher for the record), Late game is determined more by positioning than actual mechanical skill, if the Syndra player overextends she will die since she's made out of paper, on the other hand if the adc overextend, Syndra will blow him up.
Consider then that actually nuking someone requires at least 5 spheres, so she must cast Q twice, that's 160 mana, if she wants to keep the oldest sphere alive she must cast her W which is 100 mana, for a total of 260, pretty good so far, her ult is also 80 mana at lvl 18 so it goes up to 340, not too shabby considering that you get 400 mana from morellonomicon.
this can only happen in the span of around 10 seconds without cdr (with 40% it goes down to 6 ish) so as you can see, Syndra can't choose to nuke someone, she needs to be ready for it when a window of opportunity shows up, this means that every 4 seconds (3 sec with 40% cdr) she must be casting her Q with or without a target
1400+400 = 1800
1800/80= 22.5 casts of Q available with her mana pool + morello
considering mana regen you can bump that number up to maybe 25 casts of Q before she depletes her mana, but you gotta subtract the mana cost of her other skills since she needs to use them in order to get close, to keep her spheres alive and to cast her ult:
W = 100
E = 50
R = 80
1800 - 230 = 1570
1570/80 = 19.625
lets round that up to 21 available casts with mana regen, still pretty cool right?
21 x 3 secs of cd= 63 seconds in total
If Syndra SPAMS her skills like crazy, all you need to wait is 1 minute and 3 seconds before she's oom. This of course is only taking in consideration Morellonomicon, at lvl 18 Syndra has a lot more than that and these numbers inflate quite a bit. The point is though, a Morellonomicon alone won't solve your mana problems.
If by a miracle you have Morellonomicon at lvl 6:
Level 3 Q: 60 (two casts = 120)
Level 1 W: 60
Level 1 E: 50
Level 1 R: 120
120+60+50+120 = 350 total mana for a full "nuke" combo
Mana pool at lvl 6: 621 + 400 = 1021
take mana from W, E and R:
1021 - 60 - 50 - 120 = 791
Casts of Q available:
791/60: 13.18
15 casts of Q taking in consideration mana regen,
Time until Syndra goes oom if she spams Q:
15 x 4 secs of cd = 60 seconds
Considering that in lane she needs to farm and will use Q occasionally, she won't be topped off in mana 100% of the time, so you can take 1/3 of those numbers away. It is NOT that oppressive and morello will of course help but won't solve all her problems, she IS heavily mana gated considering she needs to keep her spheres rolling if she wants to be safe.
It's simple, play safe until she has less than half mana, avoid her stun to the best of your ability and don't get harrassed for free, if you engage her when she has more than 1 sphere, you deserve to loose the trade and even potentionally die to her ult, you KNOW that at that point she's the strongest and you still choose to fight anyway.