As far as I understand, this structure is grammatically correct:
Not only would it provide …, but it also would…
Can we omit "but" without introducing a mistake?
Not only would it provide …, it also would…
I've found the following explanation here but I think this example has slightly different meaning:
Usually, a "not only" feels lost without a "but also" to pal around
with. However, it is idiomatically possible and acceptable to omit the
"but also." Burchfield gives this example: "Rowers not only face
backward, they race backward." Authority: The New Fowler's Modern
English Usage edited by R.W. Burchfield. Clarendon Press: Oxford,
England. 1996. Used with the permission of Oxford University Press.
(under "not")
Best Answer
Omitting but leads to a nasty comma splice. But's role as a coordinating conjunction is to join those two independent clauses. You could, however, use a semicolon:
In my opinion, the quoted example ("Rowers not only face backward, they race backward.") is grammatically incorrect. I would use a semicolon or include but before they.