SE, the 7.0 BLM changes will not increase engagement. They are completely-pointless.

[Ferrinus]

(Emphasis mine, but I think you meant "Flare Star" there)
The heart of what you're saying is technically true, but in EW you can easily recoup from a "lost" Fire IV (that's the whole reason why you might want to cut your AF short on purpose). Usually, when you do a line like 4xF4>Despair many times, the proportion of Paradox and Despair casts overall increases (well, it depends on the phase/fight duration, but in general this is true). In fact, people sometimes do that to maximize Despairs on short phases. My point here is that the Fire IV you "lost forever" usually means an extra Paradox or Despair when you tally your gcds at the end of that phase. That might be a small potency loss (since Fire IV is more potency than Paradox, and usually you lose 2 Fire IVs for one Despair+Paradox), but it's usually not a big deal.
In DT, since you also lose the Flare Star, it is a pretty big deal. At the end of the day, you can only fit N gcds in a phase, and in EW, if you know what you're doing, the lost Fire IVs will correspond to some combination of procs, Paradoxes and Despairs.
In DT, if this happens, you just lose a Flare Star, period.

Whoops! Yeah, I meant Flare Star.

I guess what I'd point out here is that there's a "lost" Fire IV and a lost Fire IV. If you want to skip Fire IVs profitably, you need to know what you're doing and plan it out to an extent, often paying for it ahead of time by skipping a Blizz IV or B3 or both ahead of time. But it's entirely possible to do all the clever nonstandard stuff, screw something up, and end up only casting three Fire IVs before your Despair, or only two, or to cast your four Fire IVs but fail to get your Despair off because you got greedy. More prosaically, you might have to cut your cycle short by casting only four Fire IVs... but, tragically, you were ready to cast six because you'd cast B3->B4 before this astral cycle, so now you're just owned because you forgot about an incoming boss AoE or something.

So, like, mistakes are mistakes. You can sometimes recoup some, but often not all, of the loss, especially not if you manage to overcap Polyglot stacks or fat-finger Convert with full MP or something.

[Taranok]

Whoops! Yeah, I meant Flare Star.

I guess what I'd point out here is that there's a "lost" Fire IV and a lost Fire IV. If you want to skip Fire IVs profitably, you need to know what you're doing and plan it out to an extent, often paying for it ahead of time by skipping a Blizz IV or B3 or both ahead of time. But it's entirely possible to do all the clever nonstandard stuff, screw something up, and end up only casting three Fire IVs before your Despair, or only two, or to cast your four Fire IVs but fail to get your Despair off because you got greedy. More prosaically, you might have to cut your cycle short by casting only four Fire IVs... but, tragically, you were ready to cast six because you'd cast B3->B4 before this astral cycle, so now you're just owned because you forgot about an incoming boss AoE or something.

So, like, mistakes are mistakes. You can sometimes recoup some, but often not all, of the loss, especially not if you manage to overcap Polyglot stacks or fat-finger Convert with full MP or something.

To add to this, joining the discussion late, I'd describe it as catastrophic losses of fire 4s. This is when you lose a fire 4 in such a way that you also abort out of the timer itself. When this happens, you lose ~3-32s of polyglot at 29.33~ potency per second lost (~1547.8 potency max currently with +30% magic and mend, +23% eno-chan.) You might ask, 'wait, 32s?' If you drop it at exactly 29s, you have to spend ~3.5s casting back in, so that's also lost potency that cannot be recovered. Then you have to spend 2000 mp casting back in, which is a DPS loss since you're likely hard-casting back into the rotation. Let's assume fire 3. That spell is ~415 potency true casting back in at relative 3.5s cast time or 118.8 pps with the highly unlikely 0 spell speed scenario. This costs you 2000 mp and will cost you a fire 4. Well, a current fire 4 is 310 * 1.8 * 1.3 * 1.23 or ~892 potency at 318pps. You're straight up losing both 892 potency from a lost fire 4, and also the difference in pps from the opportunity cost of having to hardcast back in. That MP cost likely is costing you a second fire 4 due to fire 3 costing 2000mp, but it's possible you entered AF3 with exactly 10k mana instead of 9.6k that also can happen if you thunder late and don't get a tic.

There's a lot more that goes into it than this, but there's a reason I often equate BLM losing timers to being equivalent to a NIN casting TCJ and straight up losing the entire thing to accidental movement immediately after. I know way too many people who hate BLM because of this sword of damocles constantly hanging over the head of the class. Especially as the risks get greater and greater.