I get that this is supposed to be the first/only (or at least a very early) event involving a merge of a human and a mechanical consciousness/A.I. in the Trek universe; when ST:TMP came out it was a somewhat revolutionary SciFi concept and it was presented with such a feeling of hope and resolution that the best possible outcome for the problem was achieved.
You can tell that Roddenberry was still very much in charge here, this type of ending and overall tone captures the general optimism and philosophy that made TOS a classic and made it endure while other scifi TV shows of its time faded considerably more from memory over the years.
Plus it gave Dekker's arc a resolution by having him ending up fused with what remiained of Illya somehow - yeah yeah maybe I'm being corny here, so sue me.
Having said that, what doesn't sit right with me is just how could this new entity could become the Borg?!?!? I mean those f*ckers with their hive mind/A.I. (I guess you can't call them a race) are the single most relentless, disregardful force against individuality and natural evolution.
I just find it hard to swallow going from the positive original ending and the overall good nature of this new entity's - I dont know
ingredients? - to such a bleak twist in just a matter of seconds just felt wrong to me.
Not to mention how that contradicts the common belief that the Borg originated and evolved in the Delta quadrant while TPM clearly takes place in Alpha - even tho the quadrants hadn't come up yet at that point in the series. Maybe Im nitpicking here but the sure fire way to kill suspension of disbelief is thru things such as this IMO.
But heck man, its your edit and of course you can do whatever you want with it; I just wanted to elaborate on where I'm coming from and hope you'll take it into consideration