"Not only..., but also" without "but"
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")
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:
Not only would it provide...; it also would...
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.
There is an exception to comma splices mentioned by the MLA style center, which is consistent with the New Fowler's Modern English Usage mentioned above. From the website:
Commas are acceptable in idiomatic constructions when the second part of the sentence completes the sense of the first and in a series of three or more items:
It’s not just the customers who are exasperated, it’s the employees too.
The greater the risk, the greater the reward.
We came, we saw, we conquered.
Dinner was over, the guests had departed, and all that remained was a giant pile of dirty dishes.
It seems to me this sense would apply to the 'not only, but also' without a but construction. It requires a comma but is also completing the sense of the first clause. The grammar in 'Rowers not only face backward, they race backward' in this sense is okay.
More info is in the link here: https://style.mla.org/splices/