Wow. You went down the most irrelevant of statistical rabbit holes just to say you were right. And yet...
You missed the point: The Eagles had a front man and it wasn't Don Henley. Yes, I've seen them too, as well as Henley solo, several times. The last few times I saw him, he didn't play drums at all. Still sounded great. Know why? Cuz he's Don f'n Henley. And Don f'n Henley is a great singer, an average drummer, and a zombie of a frontman. That's why Glenn Frye was the frontman for the Eagles. And as for your irrelevant point about sales figures, their albums sold because of their great songwriting, not because the drummer was the singer on half of it.
Since you missed the point, here it is again - The OP has a band. They want to fire the drummer and stick the singer behind the drum kit. He wants to know if it's okay. Of course it's okay, but it comes with a price: The reason bands don't commonly use drummers as their sole lead vocalist is because the kit is a barrier to interaction with the audience, and the nature of the instrument impedes good vocal technique. Henley is a good singer, but his drumming is run of the mill. If he played like Neil Peart, he wouldn't be able to sing like Don Henley. You have to choose.
The OP's situation is that they don't have a Glenn Frye or a Randy Meisner to take the load off the singer now trapped behind a kit. They play modern progmetal, not 70's California softrock, so there's no danger of selling multiplatinum albums via radio airplay. Your example of Don Henley is not very applicable to the OP's situation. I hope they rock on and find a lineup that best serves their music whether live or recorded. But there's a reason the very good drummer was choosing to be the singer/frontman and not the drummer.
And really, dude. Album sales? That's what you think matters?