I still think Rebirth will easily clear 5 million when all is said an done. Might even get close to that on Playstation alone (too optimistic, I know). PC and Switch 2 versions each can pass a million, and an Xbox version could add some 400k.
This game will get a number of sales boosts from ports, price drops, GOTY awards (hopefully), and a final boost when the 3rd and final part arrives.
A reminder that FF7 Remake is actually the 2nd best selling FF in the US (dollar sale) behind FF15 and ahead of original FF7. Rebirth may not match that, but it will have the advantage of having a Switch 2 version, a better word of mouth, and more room for effective pricecuts. Remake will also be ported to Switch 2, which may bring its total sales to over 8 million, and for all we know could put US totals over FF15's.