People ask for this all the time and it's really worrying because, sadly, there's a pretty good chance SE's going to do it down the line, maybe in 6.0. It would be really unfortunate because a change like that would be an
unconditional nerf to the job and make it feel even more terrible to play. Getting aetherflow replaced by a worse version of itself (energy drain) was bad enough, this would just complete the dismantling of everything that made the job good.
The entire point of DWT and summon bahamut being separate is that you get the option to use both parts separately if you need to. If the boss spawns now, at 0:00 seconds, but a bunch of adds only appear later on at 0:40 seconds, you're able to do things like use DWT at 0:00 to start the cooldown, but only use bahamut 40 seconds later when you really need it. This doesn't change the overall structure of your rotation, as FBT is still coming off cooldown at 1:00, you just get the option to summon bahamut wherever you like within that 1 minute, whether you frontload or backload it is up to you.
If anything, FBT+summon phoenix being a single button is the problem with the job, because you lose the ability to do things like the above. There is literally no advantage to combining the two buttons, all it does is remove options. If you want to pretend they're combined now there's nothing stopping you from hitting both buttons back-to-back. All it'll do is make life easier for people who have trouble with their hotbars or something - but everyone else will suffer.
Fun fact, in the latest ultimate fight, there are at least 2-3 situations where leveraging the ability to summon bahamut separately from DWT results in a massive dps gain. During the tank lb3 transition going into the final phase, it so happens that DWT is close to coming off cooldown. If you press DWT before you get stunned, you'll waste the deathflare but gain the ability to begin the final phase with summon bahamut. And because the cutscene is so long, DWT/FBT is back off cooldown by the time it ends too, which means after bahamut ends you can immediately press FBT again and follow it up with phoenix. You basically get 40 seconds straight of demi summons at the start of the last phase.
Getting an extra demi summon, up to 2000 extra potency on the only phase where dps matters is a pretty big deal, and this is only possible because DWT and summon bahamut are separate. If you tried this with FBT, you'd just summon phoenix at the same time and watch helplessly as it does nothing during the cutscene.
Sometimes a visual example is best, so here's that exact moment from my last reclear.
https://www.twitch.tv/videos/523150159?t=00h10m30s