Nah convert is a really bad ability. Something that used to be very good and synnergized with BLM's kit and became weirdly akward and weak over the course of the game changes. Remember what Convert did on the course of the game.
During ARR era convert had great synergy into our simplistic kit, we could finish our Raging Strikes window with a bang, casting one single Flare that had a huge potency advantage over Fire, followed by a Convert, a Fire then Blizzard III time. Once convert was back up and so was Raging all we had to do was wait a fresh new Fire phase to unload that somewhat free buffed Flare again.
During Heavensward the concept of that was expanded. At the start we had only ten seconds of Astral Fire and low Spell Speed so people tried to make Convert fit in the opener for a Flare+Convert stack refresh. But since Fire IV was stronger better openers eventually showed up still expanding the concept to just fit more Spells in a tighter Astral Fire with Raging Strikes window and take advantage of Fire IV's extra potency inside Raging and Foe, which was a huge gain in the end. The second use of Convert was lined up with Raging and Ley Lines so we could o the same spells in that buff window once more.
Ok, by Stormblood... Flare and Fire IV have the same potency and our Astral Fire rotation is longer, so the idea would go back to the concept of using Flare as a stack refresh right? Yeah that can work, but also not doing a Flare is a better option to minimize clipping and taking advantage of Triple+Swift to shoot 4 Fire IV in a row without the self clip of the spell for a better gain. So we're back to the second situation of Heavensward using convert for a few more Fire IVs and that is it. What does that give us in reality isn't much because we don't have raw damage buffs to pair up with it anymore. All Convert really does is, in a pratical sense of synergy with our kit, is extend our opener so we can have a Foul charged by the end of it. Ok so now middle fight when Convert is back we can simply use it right? No, if you don't control MP ticks (oh look another issue that can be caused by them) with the right spell order on Umbral Ice you will not gain two Fire IVs from a convert! Simply because the MP provided by it is not 4800 but 4644. A difference of 166 MP that you want to have in your pool to burn in one more Fire IV. On top of that making Foul potentially clip in itself by making that cycle longer mid fight instead of going from a zero charged Polyglot (though that varies on encounters) while either making stack juggling on reopening harder or forcing a Sharp Fire in it to keep stacks. Which while isn't bad it is a smaller gain over a successful Sharp Thunder. All these little things add up on making Convert akward to use and at the same time too weak for its ridiculously long recast time.
Convert was mostly powerful for the fact it was used to extend a burst inside Raging Strikes, without a damage buff the ability became weak and with our new rotation it also became more akward to use. On top of that the HP cost just makes BLM more unliked by parties' healers specially on learnings and progression, yeah you can Manaward but then we go back to the "we always clip with ogcds" issue and having to work around that is yet another job issue that was discussed in this thread). Convert should either cause an extra effect, provide slightly more MP or have its recast reduced. Or any combination of those.