If the Lightning Streak does win, Cannala is basically guaranteed to design their own turbofan engined jet fighter next turn as well. Here's why that's bad for us.
Quick recap: we introduced the Tbird, Cannala reacted next turn by making the Spearhead. In terms of actual design the Tbird is slightly superior, mainly because the Cannalans weighed down their air superiority fighter with too much armament. It's important to note that the actual jet engine tech of the two were on par, after accounting for our extra revision spent on it and the Cannalan experience with the Bull's turboshaft engine. The situation as it stands today (both fighters being equally costly) is that we have slight design advantage+Magic advantage.
Let's say we roll out the Lightning this coming turn, gaining (more) air superiority and pushing them back a square. Yay. (We could have done that with Turbohaast too). Next turn, they introduce Super Spearhead, their own turbofan engine fighter. Its engine performance will be on par with the Lightning's turbofans, considering it's both our first forays into turbofan tech. The Cannalans will not be so stupid as to commit the same mistake twice of over-arming fighters, meaning it's down to luck of the roll as to which design is better. Assuming both roll the same, we would have gone from design+Magic advantage to just Magic advantage, which is a definite downgrade.
To sum it up, the status quo fighter design-wise is already in our favor. If we want to mix it up with a new fighter, it should definitely, absolutely be better than anything the Cannalans can put out in a single turn. The way to do that is incremental design, get the Turbohaast for the turbofan engine first before the Future Jet. Even if they are tipped off and we both go for fighter next turn, ours will be better because of turbofan experience from Turbohaast.
And as a final closer, the Cannalans are still sitting on a Research Credit. They are not averse to saving it for a rainy (lightningy?) day. You are basically betting 100% that they will spend it this turn and that we will roll higher on our jet than they will roll on their jet, because if they don't, Super Spearhead will trash the Lightning so hard we're gonna need Magic to save us.