First of all, its pretty laughable that you are claiming RNG is now the problem with Spellspeed. I'm sure Crit is not RNG dependent at all.
Second of all, you say at the extreme its 0.1 Second GCD reduction. If that was true, you would be looking at 25 casts in order to get a free cast not 30. Furthermore, lets look at the Spellspeed of Puro's BIS with your favoured Crit Chestpiece, compared to my BIS. That is 490 spellspeed (2.36gcd) vs 649 (2.20gcd) Funny that, its more than what you said was an extreme. Which means to catch up it will take around 15 spells. Half of what you said. Even if you use the Spellspeed Chestpiece on Puro's set, its still larger than 0.1 Second GCD reduction.
Third of all. If you are cancelling casts as a spellspeed BLM, you are doing something wrong. I very rarely have to cancel a cast, and if I do its due to my own error. You should work your movement out of either: a) Anticipating movement with cast clipping, b) Using Aetherial Manipulation, c) Using one of your shields to ignore a mechanic, d) Use Procs to move, e) Use scathe. I have gone through many mechanics throughout this thread where this applies in each of the fights. If you know the fight and your role, you do not have to cancel your cast often.
Fourth. If you have to cancel your cast as a spellspeed build, you will have to cancel it in a crit build; however the same does not apply when reversed, which is what Numenor explained. As I pointed out above, its more than a 0.1 Second GCD reduction
Fifth: Spellspeed gives you the greatest amount of DPS point for point, regardless of seleen. The spellspeed build I posted earlier in the thread out dpses the BiS on the calculator and gives more theoretical intelligence.
If you actually think that cancelling casts happens as often as you think and that spellspeed dps is luck dependent, then all i'll say is enjoy your slower casting RNG dependent crit build.
If this is the case, then you would certainly do more with spellspeed. However, saying "we rly dont Need Maximum dmg" is a poor attitude, especially when talking about DPS numbers.
tl;dr
Takes just over 15 spells for my BiS spellspeed build to beat the BiS Crit build. Slightly more spells compared to the BiS Puro build. There are plenty of ways to avoid/prepare for movement as Black Mage. More details in above post![]()