BLM flow and instant Cast logic problems

[Rilifane]

I think you're seriously misunderstanding the basics of BLM and some things in general.
Instant casts and off globals are two completely different things and have a different purpose. Instant casts are casts that are on global cooldown BUT have no casttime - like Xeno, procs and any triple/ swift casts. They offer you a double weaving window and mobility for one gcd.
Off globals are a completely different thing. No skill in this game was ever turned from a regular cast into an off global via buff. Off globals can be weaved during your global cooldown and while the BLM doesn't have many skills that need weaving, without instant casts you'd constantly be clipping. And THAT is the actual "locking your character in animation and preventing them from casting".

Triplecast/ Swiftcast doubles as dps gain when used on casts longer than your gcd and mobility tool. The ~0,3sec you gain by having three instant Fire IV compared to casting it not only is a dps gain, it also gives you almost 1sec of additional breathing room to get more casts done before you have to refresh AF. And the mobility + 3 double weaving windows are fantastic. It's the reason why the opener works as well as it does, because you can pot, throw LL and (if neccessary) use Addle/ MW to help mitigate damage if the boss starts with an aoe.

BLM is the class with the least ogcds and weaving windows (only counting dps classes here), so asking for instant casts (which offer said windows) to be turned into ogcds (which need said windows) is a pretty good way to completely screw BLM gameplay.
The challenge of BLM is to maintain your steady rotation while keeping uptime and handling mechanics and I can assure you, it's quite possible. BLM performs really well. The only boss where I found many mechanics a nuisance was E3s.

I'm not sure what you did with your 2000++ hours of playing only BLM but leaning the basics of the class apparently wasn't part of it. And stating that people who know what they're talking about (just because they haven't tagged their main class as BLM in forums doesn't mean they have no knowledge of the class) makes you seem like a troll.
BLM definitely doesn't need any changes in gameplay. It flows and performs well, escpecially at max level and has enough tools to react to mechanics but not too many which would take away the biggest challenge of playing BLM well. Some classes have more difficult rotations but are easier to handle in practice, with BLM it's simply the opposite.